Prepare all ingredients.
Cut the cauliflower in large flowerets. Wash them in vinegar water.
Cook the cauliflower for 15 minutes in salted boiling water.
When finished, drain.
Make a roux blanc with 30 g butter and the flour.
Pour the boiling milk on the roux blanc...
...and constantly whisk using a whisk until you obtain a thick and homogeneous sauce. Add salt, pepper, and nutmeg.
Off the heat, add the egg yolks one after another, the grated Swiss cheese and parmesan. You then have a variation of the sauce Mornay.
In a buttered gratiné dish, place a coating of sauce.
Add the cauliflower,...
...coat them with the sauce.
Sprinkle the rest of mixed cheeses, breadcrumbs and a few knobs of butter on the surface.
Bake in hot oven under the grill for about 20 minutes to obtain an even gratin.