Italians and Americans both have quite different ideas of what lasagna is, but this is what Brits think it is. The name, by the way - 'lasagna' is a single sheet of the flat pasta. 'Lasagne' is the plural. But we tend to call it lasagna anyway because the English language doesn't like pronouncing unaccented e's at the end of words.
Lasagna has four main components - a meat and tomato sauce (British Bolognese Sauce), a white sauce with a bit of cheese in it, based on a classic béchamel sauce, layers of pasta sheets, and a molten cheese topping.
Now for a really deep tasting sauce, you need to cook it for 3 hours. You can do this in the casserole on the lowest possible heat on your stovetop or in the oven. Or you could do it in a slow cooker for about 6 hours. Keep the lid off so the liquid level can reduce and concentrate the flavour. Now and again, check that the sauce isn't getting too dry - add more stock or water if it is.
When cooked, taste the sauce and add salt if necessary. If the sauce is too runny (it should be moist but not sitting in liquid), turn the heat on full and reduce the excess liquid. You can use this sauce straight away, but it does improve if you leave it overnight.
You'll need enough sheets of pasta to create 3 layers in your dish. If your pasta is pre-cooked, you can use it after soaking and cutting. For uncooked dried pasta, follow the manufacturer's instructions.
Butter the insides and bottom of your dish. Spread a thin layer of the bolognese sauce on the bottom. Cover this with a layer of pasta sheets, cutting them as necessary to fit the dish, and trying not to overlap the sheets. Spread another layer of bolognese sauce, and then a layer of the white sauce. Cover this with a second layer of pasta, and cover that with the remaining bolognese sauce. Add the final layer of pasta, the remainder of the white sauce, and sprinkle the second half of the cheese evenly on top.
Vegetable oil for sautéeing. Enough dried lasagna sheets to create 3 layers in your baking dish. The cheese can be whatever you like, but preferably a flavoursome melty cheese. I use mature cheddar.