Cauliflower au gratin is so good that it's a shame not to make it whenever you can find nice cauliflower at a good price. I made this one with French lardons fumés (diced smoked pork belly) but you could use diced ham or even something like smoked turkey or chicken for good flavor. This was our lunch yesterday, and we ate almost the whole thing. I had bought a beautiful chou-fleur at the supermarket for two euros.
Cut or break the cauliflower into florets and pre-cook them in boiling water or in a steamer pot until they are just tender. Sauté the lardons, ham, or poultry with a little bit of oil or butter. Then, separately, make a sauce béchamel. That's a white sauce made with butter, flour, and milk. Add cream for a richer sauce. Make the sauce fairly thick or fairly liquid, to taste. Season the sauce with black pepper, a grating or pinch of nutmeg, and a little bit of salt.
Arrange the cooked cauliflower florets in a baking dish. Pour the sauce over them. Grate about three-fourths of a cup of Swiss or Cheddar cheese. You can add some of the cheese to the sauce, or just sprinkle it over the cauliflower and lardons before you pour on the white sauce. Save a third or so of the grated cheese to sprinkle on top of the sauce. Bake the gratin de chou-fleur in a hot oven for 20 to 30 minutes until the sauce is bubbling and starting to brown on top. Serve hot!