Heat the oven to 375°F. Rinse the Brussels sprouts, trim off any wilted outer leaves, then halve them lengthwise or quarter large ones.

Heat the olive oil in a skillet over medium-high heat. Add the sprouts and cook for 5 to 6 minutes, stirring until tender-crisp and lightly browned. Transfer to a 2-quart baking dish.

In the same skillet, cook the chopped bacon over medium-high heat until crisp. Remove the bacon, leaving about 2 tablespoons of drippings in the pan. Lower the heat to medium, stir in the flour, and cook for 2 to 3 minutes.

Gradually add the milk while whisking until smooth.

Remove the skillet from heat, whisk in the Gruyère or cheddar, onion powder, salt, and black pepper. Pour the sauce over the sprouts, add the bacon, and stir gently to combine.

In a small bowl, mix the crushed Ritz crackers, Parmesan, and melted butter. Sprinkle the topping over the casserole.

Bake uncovered for 20 to 25 minutes until the top is browned and the sauce is bubbly.

Assemble the casserole up to 2 days in advance and refrigerate. Add about 5 minutes to the baking time if it is cold.

Store leftovers in an airtight container in the refrigerator for up to 4 days. Baked leftovers can be frozen for up to 1 month.