How do you make a vegetable lasagne that isn't watery?

The classic disappointment: a beautiful lasagne comes out of the oven, you cut the first piece, and a pool of pale liquid floods the corner of the dish. The layers slide. The flavour is diluted. Every part of that problem starts with water, and most of the water comes from the vegetables.

Why vegetable lasagne goes watery

Vegetables hold a lot of water. When they bake inside layers of pasta and sauce, that water comes out and has nowhere to go. Courgettes, aubergines, mushrooms, spinach and peppers are the main offenders. Add a loose sauce and fresh sheets of pasta that can't absorb any more, and you get soup with ambitions.

Step 1: Pre-cook every vegetable

Raw vegetables should not go into a lasagne. Cook them first, until they have given up most of their water and picked up some colour.

• Courgettes and aubergines: slice and roast or grill until browned and somewhat shrunken. Salting first can help with courgettes; see watery courgettes and salting aubergine.
• Mushrooms: fry in a hot pan, not crowded, until the liquid has evaporated and they're golden.
• Spinach and greens: wilt, cool, then squeeze very hard in your hands or a clean tea towel. You'll be surprised how much water comes out.
• Peppers: roast and peel, then drain on kitchen paper.
• Squash or pumpkin: roast rather than boil.

Roasting and grilling do double duty: they remove water and add flavour, so the lasagne tastes of browned vegetables rather than steamed ones.

Step 2: Thicker sauces

Tomato sauce

Simmer it until it is thick and jammy, so a spoon dragged through the pan leaves a clear trail. A loose, watery sauce in the pan becomes a puddle in the dish.

Béchamel

Make it thicker than for other uses, like thick custard, coating the back of a spoon heavily. It will loosen as it bakes and absorbs moisture from the vegetables.

Ricotta

If using ricotta, drain it in a sieve for a while first, and mix it with an egg and grated cheese for a filling that holds together.

Step 3: Layering

• Start with a thin layer of sauce on the bottom so the first sheets don't stick.
• Layer thinly and evenly: pasta, a thin layer of vegetables, a little tomato, a little béchamel, cheese. Many thin layers set better than a few thick ones.
• Spread the vegetables right to the edges, so the layers are even.
• Don't overfill. Leave a little room at the top of the dish.
• Finish with béchamel and cheese on top so the surface browns and seals.

Step 4: The pasta

Dried lasagne sheets absorb liquid as they cook, which can help with a slightly wet filling. Fresh sheets absorb less. If using no-cook sheets, follow the packet guidance on sauce quantities, and if your filling is very dry, add a little more sauce. The aim is balance: enough moisture for the pasta to cook, not so much that it pools.

Step 5: Bake, then rest

Bake until the top is golden and bubbling at the edges. Then the most important and most ignored step: let it rest. Leave it out of the oven for a good while before cutting, at least long enough for the bubbling to stop completely and the dish to be warm rather than scalding. As it cools slightly, the sauces set and hold the layers together. Cut it straight from the oven and it will slide; wait and it will stand.

Lasagne is even easier to cut neatly the next day, reheated, which makes it a good dish to make ahead.

If it's watery anyway

• Tilt the dish and spoon off excess liquid from a corner.
• Rest it longer; it often firms up.
• Return it to the oven, uncovered, to evaporate some liquid.
• Make a note of which vegetable was the culprit for next time.

Similar rules apply to other baked dishes; see pasta bake that isn't dry for the opposite problem, and parmigiana di melanzane for a close relation.