1 lb. smoked bacon, diced
1/2 cup finely diced onion
1½ Tbs. minced garlic
1 Tbs. all-purpose flour
1/4 cup dry white wine
1 cup heavy whipping cream
1 Tbs. finely diced fresh thyme leaves
1 lb. Brussels sprouts, shaved thin, preferably on a mandoline
1/2 cup freshly grated sharp cheddar cheese
1/2 cup freshly grated fontina cheese
Kosher salt and freshly ground black pepper
1 cup bread crumbs
1/2 cup Parmesan cheese
2 Tbs. extra-virgin olive oil
Preheat the oven to 350°F.
Put the bacon in a large frying pan and cook over medium heat until crispy. Reserve the fat. Drain the bacon on paper towels.
Put the reserved bacon fat, onion, and garlic in a saucepan and warm over medium-high heat until fragrant, about one minute. Stir in the flour and cook for two minutes, stirring. Whisk in the wine, cream, and thyme and bring to a light simmer. Remove from the heat and pour into a large bowl. Add the Brussels sprouts, cheddar, fontina, and bacon. Toss to combine. Add salt and pepper to taste. Spread the mixture in a nine-inch by 13-inch casserole dish.
Combine the bread crumbs, Parmesan, and olive oil in a separate bowl and sprinkle on top of the Brussels sprouts mixture. Place the casserole dish in a larger baking dish deep enough to hold hot water halfway up the sides of the casserole. Slide the baking dish into the oven and pour in the hot water. Bake the gratin for 30 minutes, or until it is hot through, the top is golden, and the interior is creamy.