I lived in Brussels for a summer, renting a floor from a nice couple (Flemish girl and Turkish guy). When we discovered we share a food hobby, she started taking me on grocery shopping trips and by the end of my stay, I learned a bunch of local recipes. Later I wanted to cook one of the most typical (Witloof in den Oven) for a friend, but he was vegetarian (the traditional recipe calls for the endive heads to be wrapped in ham), and truth be told, I wasn't so crazy about Belgian endives anyway, so I tried a riff on the same idea. I replaced the bitter endive with something equally "Flemish" (Brussels sprouts were indeed first grown in Brussels), and the meat with mushrooms. For the rest I essentially retained the wonderful and easy Béchamel-Mornay-type base, but was looking for something to add more flavor, and so - the roasted garlic cloves interspersed in the casserole which my friend called "hidden jewels." —Dessito
Roasted garlic cloves
heads of garlic (if average size; you basically want 15-20 cloves)
Brussels sprouts (this is a dish which does not suffer if you use frozen ones)
butter (I use salted)
regular white flour
nutmeg (freshly ground is definitely better here)
grated Gruyere (or Emmental, or even old Kashkaval, or a combination)
Pre-heat your oven to 350-400F. Cut the whole stem-side tip of each head of garlic, so that the raw garlic shows under the papery skin, but without wasting too much of the good stuff. Generously rub flavorful olive oil over the cut. Wrap each head in a pouch of aluminum foil (set root-side down in the middle of a square piece of foil which, when pressed up around the head covers it completely on each side and has enough slack left to bunch up on top). Leave a small opening in the folded foil over the center of the stem-side for stem to escape through while roasting. Drizzle a few more drops of oil through this opening. Place the two well wrapped heads with the openings up on a rack in the middle of your oven and let them roast for 45 minutes while you are preparing the rest of the dish.
If using fresh sprouts, cut the stems with a V-cut (to aid cooking later and to allow for their flavor to develop more fully) and clean any yellowing leaves. If using frozen sprouts, you can make a shallow X-cut in each head for the same purpose, without loosening too many leaves.
Saute the mushrooms in the butter over medium heat for 2-3 minutes. Turn up the heat to high-medium, add the Brussels sprouts and stir around until they are well covered with butter and are starting to cook, about 3 minutes.
Add the flour and spices, again stirring well. Cook the flour for another couple of minutes, stirring frequently. While all of the mix will ultimately finish cooking in the oven, you want to make sure the flour's raw taste is gone at this point, which needs at least 1-2 minutes. Do not allow the flour to brown.
Add the milk little by little, stirring so that the cooked flour does not lump in the liquid. Bring to a simmer, then lower the heat again. In a few more minutes the sauce thickens visibly (you should be stirring it occasionally).
Stir in 1 cup of the cheese until it melts. (If you think the sauce is too thick, you can add another 1/2 cup milk, but generally the juices of the mushrooms and sprouts release in the first step provide sufficient addition to the specified amount of milk).
Squeeze the garlic cloves out of their skins (which you have taken out of the oven after 45 or so minutes of roasting and ideally also cooled down by now) into the mix. If roasted sufficiently, they should pop out very easily.
Pour everything in a flat dish which will go in the oven and sprinkle an even layer on top with the remaining 1/2 cup of cheese. My personal favorite combination is Gruyere in the mix and Bulgarian Kashkaval (preferably a piece that has been in the fridge for some time, so that it gets hard enough for grating) for the top crust since it not only melts nicely but turns a wonderful golden-brown color under the broiler.)
Cover the casserole with aluminum foil and place in the middle of the pre-heated oven. Bake for 20 minutes. Remove the foil and switch to the broiler for the last five minutes. The dish is great to eat immediately, but also re-heats very well a day, two or three later.