Blending is a decision, not a finish
Most recipes treat blending as the last step, something you do to a soup once it is made. It is better understood as a choice that makes two different soups out of the same pan, and it is worth making that choice deliberately rather than reaching for whatever is in the drawer.
A stick blender, worked for a minute. Flecks of carrot stay visible, the texture keeps some body, and it eats like a broth with substance in it. Good on a day when the soup is the whole meal and you want something to chew.
A jug blender, run for two. The lentil starch emulsifies completely and what comes out coats the back of a spoon and looks like it has cream in it. It has not. Good as a first course, or in a mug, or when the texture is the point.
Neither is more correct. What they share is a failure mode, and it happens before either one is switched on.
Grit is undercooked carrot and it cannot be blended out
A carrot with any bite left in it does not puree. It shatters, into fragments small enough to get through the blades and large enough to feel on the tongue, and once that has happened running the blender for another minute makes no difference at all: the fragments are already smaller than the blender can usefully act on.
The test is a knife, and the bar is higher than it feels. It should slide into a chunk with no resistance whatsoever, well past the point where you would be happy eating that carrot as a carrot. Twenty five minutes at a simmer usually does it; big chunks take longer, which is the argument for cutting them roughly the same size.
If you have ever made a blended vegetable soup that was somehow both smooth and slightly sandy, this is what happened, and it is almost never the blender's fault.
Twenty seconds of dry heat
Toasting the cumin before anything else goes in the pan is the single biggest improvement available on this page, and it takes less time than reading this paragraph.
A whole spice holds its aromatic compounds locked up in a dry seed. Dry heat drives out the last of the moisture and begins breaking those compounds down into the ones you can actually smell, which is why the pan goes from smelling of nothing to smelling of cumin quite suddenly. That moment is the cue, and about ten seconds past it is burnt.
Do it in the dry pan before the oil, tip it out so it stops cooking, and crush it roughly. Ground cumin straight from a jar works and is a noticeably flatter soup, because the grinding happened months ago and the volatile compounds have been escaping ever since.
The hazard nobody mentions
Blending hot soup in a sealed jug is the most genuinely dangerous thing on this page and it is usually left as an unqualified instruction. Hot liquid gives off steam, steam in a sealed container expands, and the pressure lifts the lid. What follows is a ceiling covered in boiling soup and, occasionally, a scald.
Fill the jug half full at most. Take the centre cap out of the lid so the steam has somewhere to go, hold a folded tea towel loosely over the hole, and start on the lowest speed before working up. Or wait ten minutes and blend it warm rather than hot, which removes the problem and costs nothing but the ten minutes.
A stick blender in the pan has none of this problem, which is a decent argument for owning one if you make soup often.
Salt early, taste late
Lentils absorb a surprising amount of salt and they need time in contact with it, so it goes in at the start rather than at the end. A thick, starchy soup also presents salt differently from a thin broth, and the same amount reads as less.
Then taste after blending, because the texture change moves the perception again, and expect it to want a little more. If it tastes flat rather than under-salted, the thing missing is acid: a squeeze of lemon does more than another pinch, and it is the correction people reach for last when it should be second.
Common questions
Why is my blended soup gritty?
The carrots were not cooked enough, and no amount of further blending will fix it. A carrot that still has any bite left shatters into small hard fragments rather than breaking down into puree, and those fragments are what you feel on the tongue. Cook until a knife slides through with no resistance at all, which is noticeably past the point where you would want to eat one, then blend.
Do you have to peel carrots for soup?
No, and it is worth not doing. A good share of the flavour sits in and just under the skin, and it blends to nothing anyway, so peeling costs you taste and gains you about ten minutes of work across a year. Scrub them properly instead. The exception is a carrot that has been in the fridge long enough to go rubbery and bitter at the surface, which no technique rescues.
Should you toast spices for soup?
For twenty seconds, yes, and it is the biggest single improvement available here. Dry heat drives out moisture and starts breaking down the compounds in the seed that carry flavour, so what you get afterwards is rounder and much less raw. The cue is smell: when the pan starts smelling of cumin rather than of nothing, it is done. Ten seconds later it is burnt, which is the whole risk.
What is the difference between a stick blender and a jug blender?
How completely they break things down, and it makes two different soups. A stick blender in the pan leaves flecks and some body, so the result eats like a broth with substance in it. A jug blender run for a couple of minutes emulsifies the lentil starch completely and gives something velvety that coats the spoon with no cream involved. Neither is better. Pick the one that suits the day.
Is it safe to blend hot soup in a jug blender?
With care, and this is the one genuine hazard on the page. Hot liquid in a sealed jug produces steam, the steam expands, and the pressure can blow the lid off and throw boiling soup across the kitchen. Fill the jug no more than half, remove the centre cap from the lid, hold a folded tea towel over the hole, and start on the lowest speed. Or let the soup cool for ten minutes first, which removes the problem entirely.
Why does my soup need so much salt?
Because lentils absorb it and because a puree presents salt differently from a liquid. Pulses soak salt up as they soften, so a soup thick with lentils genuinely needs more than a thin broth of the same volume. Add it early enough to dissolve and season, then taste at the end and expect to add a little more. If a soup tastes flat but not bland, it usually wants acid rather than salt: a squeeze of lemon.
How much does a bowl of this cost?
It is the cheapest cooked dinner in this section. A bag of carrots, an onion, a cup of red lentils and stock made from a cube, and it feeds four. This tool works out cost a head from a real shopping basket with a date on it, and until those prices are entered it shows nothing at all rather than guessing, because a cost figure invented from thin air is worse than no figure.
Can you freeze carrot and lentil soup?
Three months, and it comes back as good as it went in, which is not true of every soup. What breaks in a freezer is dairy, which splits, and potato, which goes grainy as the starch recrystallises. There is neither in this one, so it is one of the few soups worth making in a genuinely large batch. Add the swirl of milk or yoghurt when you serve it rather than before it goes in.