With its creamy cheese sauce and accent of bacon, this decadent recipe may tempt even the green-vegetable averse. It can be made early in the day, refrigerated, then baked before mealtime. (If you're simply not a sprout fan, you can substitute one very large head of cauliflower, cut into bite-size florets.)
Heat a large skillet and crisp-cook the bacon. Transfer it to a plate and set it aside. Pour off all but 2 tablespoons of bacon fat from the pan. Add the onion to the pan and saute over medium heat for 6 to 7 minutes; add the garlic for the last minute. Scrape the onions and garlic into a large, shallow casserole and spread them around evenly. Set aside.
Trim the Brussels sprouts, then cut them in half (or quarters, if very large) lengthwise. Bring a large soup pot of well-salted water to a rolling boil. Add the Brussels sprouts. Return to a boil and cook for 5 minutes. Drain immediately in a colander for 5 minutes. Transfer to the casserole, spreading the sprouts evenly in the dish. Set aside. Heat the oven to 375°.
Combine the milk and chicken broth in a small saucepan. Bring to a simmer, then remove from the heat.
Meanwhile, melt the butter in a large saucepan. Add the flour and cook, stirring over medium heat, for about 1 minute, until the flour is light golden brown. Whisk in half of the hot milk and broth. Heat and whisk for a minute or so, until thick. Add the remaining liquid and cook for about 5 minutes, whisking frequently.
Whisk in the Parmesan cheese, mustard, salt, thyme, and pepper. Pour the sauce evenly over the Brussels sprouts. Mince the bacon and sprinkle it over the sauce. Sprinkle evenly with the Cheddar cheese and dust lightly with paprika, if desired.
Place the casserole on the center oven rack and bake for 30 minutes, until bubbly and golden brown. Cool for about 5 minutes before serving. Makes 10 servings.