Preheat oven to 400°F. Spray a 9x13-inch baking pan with nonstick spray and set aside.
Bring a large pot of salted water to a boil. Add in all the Brussels sprouts and cook for 6-7 minutes, or until they have turned bright green and are slightly soft. Remove the sprouts using a slotted spoon and place them in the prepared baking pan.
1½ pounds Brussels sprouts
While the Brussels are cooking, make your sauce: melt the butter in a medium saucepan over medium heat. Once melted, whisk in the flour and allow to cook for 1 minute, whisking constantly.
3 tablespoons salted butter, 3 tablespoons all-purpose flour
Whisk in your milk slowly, again whisking constantly until a sauce forms.
2 cups skim milk
Reduce heat to simmer and allow to cook for 5-6 minutes or until its thick and smooth. Whisk frequently while it cooks.
Season the salt liberally with salt and pepper to taste, and then stir in the Parmesan cheese.
Kosher salt and freshly ground black pepper, ½ cup freshly grated Parmesan cheese
Sprinkle the bacon crumbles over the brussels in the baking dish, reserving a bit for garnish.
8 slices crumbled cooked bacon
Pour the white sauce over the brussels, making sure to get in all the nooks and crannies.
Top with the shredded white cheddar and remaining bacon.
1 cup freshly shredded white cheddar cheese
Bake for 10-15 minutes or until cheese is fully melted and bubbly.