Cut the cauliflower from the stem, peel the potatoes and cook both in salted water until soft.
For the bechamel sauce: melt the butter in a saucepan, add the flour and stir quickly with a whisk so that there are no lumps. Before the butter and flour turn brown, add the milk little by little and stir quickly until the whole thing starts to boil. Leave to swell for 20 minutes over low heat and stirring frequently. Season with salt, pepper and nutmeg and set aside.
Cut the bacon into pieces about 2 cm long. Put the potatoes and cauliflower in a baking dish. Spread part of the bacon on top. The becha flour sauce, so that everything is covered as much as possible. And cover everything with the cheese. Spread the rest of the bacon on top. Cut the tomato into thin slices and finally distribute on the casserole. Put the casserole in the oven at 200 ° C for about 20 minutes.