Cook the pasta in a saucepan with salted water according to the instructions on the packet.
Meanwhile, dice the onion and sauté in part of the butter with the thyme. Add the rest of the butter and bring to the boil briefly. Stir in the flour and sauté briefly. Add the milk and bring to the boil. If the sauce is too thick, add more milk.
Season the finished sauce with salt, pepper, Worcestershire sauce and the juice of the lemon. Remove the saucepan from the hob and stir in half of the cheese.
Put the finished pasta in a baking dish and mix with the cheese sauce. Spread the rest of the cheese over it. Crumbly crush the rusks and sprinkle over them.
Bake at 250 ° C top / bottom heat in the preheated oven for about 15 - 20 minutes on the middle rack, until the rusk is golden brown.