Two things that look like mistakes
This dish has a reputation for coming out heavy, dry and slightly gluey, and it is almost always the same two errors made together. Both of them are errors of doing the sensible thing.
The pasta comes out of the water three minutes early. Not al dente: properly underdone, with a hard core you would send back in a restaurant. It is about to spend half an hour in a hot oven surrounded by liquid, which is a second cooking, and pasta that was right when it went in is mush when it comes out.
The sauce is left too thin. Loose enough that it looks like you have made a mistake with the milk. The pasta drinks a serious amount of liquid while it bakes, and the starch it gives up thickens whatever is left, so a sauce that coats the spoon nicely in the pan sets into something closer to a cake in the dish.
Do the sensible thing on both counts, which is what everybody does the first time, and the two errors compound: overcooked pasta in a sauce that has nothing left to give it.
A white sauce is a minute and then patience
Two failures, both easily avoided and both very common.
It tastes of flour. The roux was rushed. Flour stirred into melted butter needs a full minute over the heat before any liquid arrives, stirring throughout, and by the end of it the pan should smell faintly biscuity rather than of nothing. That minute cooks the raw starch out. Skip it and you get a chalky, pasty background taste that more cheese buries rather than removes.
It goes lumpy. Cold milk hit hot flour paste too fast. What happens is physical rather than careless: the outside of a lump gelatinises on contact and seals dry flour inside, where stirring cannot reach it. Add the first few splashes slowly and beat each one smooth before the next, then pour more freely once it is loose.
If it does go lumpy, a stick blender fixes it entirely in about five seconds and nobody will ever know. That is worth remembering, because the usual response is to start again.
Fold the tuna, do not stir it
Tuna out of a tin is already in flakes and those flakes are the texture of the dish. Stirred vigorously into a warm sauce it breaks down into a uniform paste, and what you get is a pasta bake that tastes faintly of fish rather than one with tuna in it.
Fold it through with a spatula, cutting down and turning over rather than going round. Half a dozen turns is plenty, and a few streaks of unmixed sauce are better than a uniform mixture that has been worked to death.
Drain it hard first, pressing with the lid of the tin. Whatever liquid comes into the dish thins the sauce you just spent effort getting right. Use tuna in water or brine rather than oil: there is already butter and cheese in here and the oil makes it heavy.
Five minutes standing
Straight out of the oven the sauce is still liquid and the bake is held together by heat. Cut into it then and it slumps across the plate, which is the standard complaint about this dish and is entirely a timing problem.
Five minutes on the side and the starch sets, the sauce thickens, and a spoon lifts out a portion that keeps its shape. It is the same five minutes a lasagne needs and it is skipped for the same reason: the thing is visibly ready and people are already at the table.
Where the fifty pence goes
There is nothing expensive in this dish. A tin of tuna, some pasta, milk, a bit of cheddar and two things out of the freezer, and it feeds four with leftovers for a lunchbox. The white sauce is flour and milk, which is the cheapest way there is of turning a small amount of cheese into a lot of sauce.
That is also why it takes so well to being doubled and frozen unbaked. Nothing in it is delicate: there is no cream to split, no fresh herb to brown, no pasta that has not already been deliberately undercooked. Assemble two, bake one, and the second is a dinner that needs no thought at all in three weeks' time.
Common questions
Should you undercook pasta before baking it?
Yes, by about three minutes off the packet time. The pasta is going to spend half an hour in a hot oven surrounded by liquid, and that is a second cooking. Pasta that was correctly done when it went in comes out soft and shapeless. Drain it and run cold water over it to stop the residual heat carrying on cooking it while you make the sauce.
Why is my pasta bake dry?
The sauce was the right thickness in the pan, which means it was too thick for the dish. Pasta absorbs a serious amount of liquid as it bakes, and the starch it sheds thickens whatever is left. So the sauce should look distinctly looser than you want the finished bake to be, to the point where it feels wrong. That is the correction, and getting it and the pasta timing wrong together is where this dish earns its reputation.
How do you stop a white sauce going lumpy?
Add the milk slowly at the start and beat it smooth before each new pour. Lumps form when cold milk meets hot flour paste too quickly: the outside of each lump gelatinises instantly and seals the dry flour inside, where no amount of stirring reaches it. A whisk is much more forgiving than a wooden spoon. If it does go lumpy, a stick blender fixes it completely in about five seconds.
Why does my white sauce taste of flour?
The roux was not cooked long enough. Flour stirred into melted butter needs a full minute over the heat, stirring, before any liquid goes near it, and it should smell faintly biscuity by the end of that. Skipping it leaves raw starch in the sauce, which has a distinctive chalky, pasty taste that more cheese covers up rather than removes.
Can you make tuna pasta bake ahead?
Assemble it fully and refrigerate for a day, or freeze it for two months, in both cases before baking rather than after. From the fridge it wants an extra ten minutes. From frozen, defrost it properly first: a frozen block the depth of a bake will not get hot in the middle before the top has burnt, and this is a dish where the middle being hot matters.
What tuna should you use?
Tuna in spring water or brine, drained hard. Tuna in oil brings its own fat into a dish that already has butter and cheese in it, and the result is heavy. Drain it properly by pressing it in the tin with the lid, because the liquid left behind thins the sauce you just spent effort thickening. Fold rather than stir it in, so it stays in flakes instead of breaking into paste.
Why does my pasta bake collapse when I serve it?
It was cut too soon. Straight out of the oven the sauce is still liquid and the whole thing is held together by heat rather than structure. Five minutes standing lets the starch set and the sauce thicken enough to hold a cut edge. This is the same reason a lasagne is left to stand, and it is the same five minutes everybody skips because it is ready and people are hungry.
Do you need breadcrumbs on top?
They are doing a texture job rather than a flavour one. The dish underneath is uniformly soft, so a crisp layer is a genuine contrast rather than a garnish. Cheese alone browns and goes chewy rather than crisp, which is a different and also fine result. Crushed crisps work, panko works better than fresh crumbs, and a mix of crumbs and cheese gives you both.