This brussels sprouts gratin is an excellent choice for an everyday meal or holiday dinner. A rich fontina cheese sauce and buttery fresh bread crumbs bring out the naturally sweet flavor of the brussels sprouts in this delicious side dish. Cheddar cheese is a good alternative to the fontina cheese, so don't hesitate to substitute if necessary. Crumbled bacon is called for, but feel free to skip that ingredient if you need a vegetarian-friendly casserole. Pancetta or 3 or 4 tablespoons of finely diced ham would also be nice additions if you'd like to add some smoky flavor.