Prepare. Preheat your oven to 350. Also, prep your casserole dish for nonstick.
Bacon. Cook your diced bacon in a large skillet over medium heat until crisp, stirring and flipping it as needed (about six to seven minutes). Carefully remove the bacon bits using a slotted spoon and transfer them to a stack of clean paper towels to drain. Also, carefully remove all but two tablespoons of the bacon fat from the skillet and save the remaining fat to use for the sauce.
Sprouts. Saute the sprouts in the in the skillet with the remaining bacon fat until softened (about six to seven minutes). Then remove the sprouts and set them aside while you work.
Roux. To make the sauce, combine together two tablespoons of the bacon fat with the flour in your skillet, whisking continuously for one minute. Slowly whisk in the milk to the skillet and continue whisking and cooking until it begins to bubble and is smooth ( about three to five minutes).
Cheese. Remove the skillet from the heat and stir or whisk in your cheese and seasonings. When the cheese has fully melted you can add the sprouts and bacon and mix until fully coated with the sauce. Lastly, evenly spread the ingredients from the skillet into your prepared casserole dish.
Topping. To make the topping, combine together the melted butter with your breadcrumbs. Then spread the topping over the ingredients in the casserole dish.
Bake. Bake your casserole for 18-20 minutes or until the brussel sprouts are crisp-tender and the breadcrumbs are golden brown.