The case against adding things
Nearly everybody's tomato sauce has more in it than this one. Garlic, dried oregano, a chilli, a splash of wine, a stock cube, a bay leaf. Each addition looks like an improvement at the moment it goes in, and the cumulative result is a sauce that tastes busy and of nothing in particular.
What actually makes a tinned tomato taste good is time at a low temperature. Forty minutes of bare simmering breaks the fruit down, drives off water, and concentrates sugars that were in the tin all along. That is a bigger change than any ingredient you could add, it costs nothing, and it is the thing people skip because it is not a decision, it is just waiting.
The test is what the sauce tastes of. If the answer is tomato, and you want more of it, that is a sauce working. If the answer is a list, something has gone in that did not need to.
The onion goes in whole and comes out whole
Halve it, put both halves in cut side down, and forget about them until the end. Over forty minutes they release their sweetness into the sauce and take nothing back, and then you lift them out and throw them away or eat them standing at the hob, which is what actually happens.
This is a different decision from chopping an onion in, and it is worth understanding as a choice rather than a shortcut. Chopped onion stays: it brings bulk and texture, and the sauce becomes about onion and tomato together. Halved onion flavours and leaves, and what you are left with is clean and unmistakably of one thing.
Clean is harder to achieve than it sounds. It is also the reason this sauce works under almost anything: on pasta, under a piece of fish, as the base of a bake, spread on a pizza. A sauce with a strong opinion about oregano is much less useful.
Butter, and why it is in an Italian sauce
Butter in a tomato sauce reads as a mistake to anybody who assumes Italian cooking means olive oil throughout. Northern Italy has always cooked in butter, and it does something here that oil does not.
Fat carries flavour compounds that water cannot, and butter's are dairy compounds that round off acid and give the sauce a softness at the edges. Olive oil is sharper and fruitier, and it pushes the sauce in the opposite direction, towards brightness. Neither is wrong. Using both, which is what this recipe does, gets a little of each.
If you want to understand the difference, make it once with all butter and once with all oil. It is a bigger change than any herb.
Stir it three times, not thirty
The instinct is to keep a pan of tomatoes moving so nothing catches. Resist it. A little catching on the base and being stirred back in is where a good part of the depth comes from, in exactly the same way that the browned bits in a roasting tin make gravy.
Three stirs across forty minutes is about right, crushing the tomatoes against the side of the pan as you go. What you are avoiding is a burnt layer rather than a browned one, and the difference is the heat: a bare simmer where a bubble breaks every second or two will brown and will not burn.
Leave it uncovered. A lid traps the steam and the sauce never reduces, so you finish with something thin that tastes of raw tin.
Make four times as much
There is no more work in four tins than in two, and this sauce is the most useful thing you can have in a freezer. It is a pasta sauce, a pizza base, the liquid in a bake, the start of a soup and the thing you pour over a piece of white fish with olives in it.
Freeze it flat in bags rather than in tubs. A sheet of frozen sauce defrosts under a warm tap in a few minutes and a solid brick out of a tub takes half an hour or a microwave, which matters more on a Tuesday than it sounds like it should.
Common questions
Why does my tomato sauce taste acidic?
Because it has not cooked long enough. Tinned tomatoes are sharp out of the tin and what softens them is time at a low temperature, which breaks down the fruit and concentrates the sugars already in it. Twenty minutes is not enough and forty usually is. A half teaspoon of sugar is a legitimate correction if the tomatoes were poor, but reach for it after the forty minutes rather than instead of them.
Should you put sugar in tomato sauce?
Only if it needs it, which you can only know by tasting at the end. Tinned tomatoes vary enormously between brands and seasons, and a good tin cooked properly needs nothing. A half teaspoon across four servings balances acid without reading as sweetness. If you find yourself adding a tablespoon, the problem is either the tomatoes or the cooking time and the sugar is covering it up rather than fixing it.
Why put a whole onion in and take it out?
Because it is flavouring the sauce rather than being an ingredient in it. A halved onion sitting cut side down gives up its sweetness slowly over forty minutes and leaves the sauce clean and unmistakably of tomato. Chopped onion does something different: it stays in, it brings texture and bulk, and the sauce becomes about two things instead of one. Both are valid and only one of them is this recipe.
Are whole tinned tomatoes better than chopped?
Usually, and the reason is unglamorous. The best fruit goes into the tins where you can see it whole, and tomatoes that were bruised or misshapen get chopped, where nobody can tell. Whole plum tomatoes crushed by hand in the pan also give you a better texture, because machine chopping produces uniform cubes that hold their shape and never quite break down. It is the same money for a noticeably better sauce.
How long can you keep tomato sauce?
Five days in the fridge, and longer if you pour a film of olive oil over the surface, which keeps air off it. Three months in the freezer. Freeze it flat in a bag rather than in a tub: a flat sheet of sauce defrosts in a few minutes under a warm tap, and a solid block in a tub takes half an hour or a microwave.
Can you use fresh tomatoes instead of tinned?
You can, and outside of a hot summer the tin is better. Tinned tomatoes are picked ripe and processed immediately, and supermarket fresh ones are picked hard so they survive being moved about. If you do use fresh, you need about twice the weight because of the water, and it is worth skinning them, which means a cross cut in the base and thirty seconds in boiling water.
Do you cook the garlic before adding tomatoes?
It depends which sauce you are making, and this one avoids the question by not using any until the swap. Sliced garlic fried first goes sweet and nutty and stays in the sauce, which is one dish. Whole cloves crushed flat and simmered in and fished out later give a rounder background note, which is another. What does not work is garlic added early over a high heat, because it burns in under a minute and burnt garlic is bitter through the whole pan.
What pasta goes with tomato sauce?
Almost anything, and shapes with texture hold it best: penne, rigatoni, fusilli. A smooth sauce and a smooth pasta slide off each other, which is why cheap dried spaghetti with a thin sauce ends up as a plate of pasta with a puddle at the bottom. Whatever you use, finish it in the pan with a splash of the cooking water rather than serving the sauce on top.