Sauté the onions with the garlic. Add the flour and cook for 1 minute. Add the milk to thicken with the flour, onions, and garlic. Combine the milk mixture with the rest of the ingredients. Top with the Ritz crackers. Bake, and serve!
Make this entirely in advance, then bake right before serving! This recipe can be made with fresh broccoli florets for an equally delicious broccoli casserole.