Prepare the casserole.Combine the casserole ingredients in a large bowl. Set aside.
Prepare the Béchamel Sauce.Melt the butter in a heavy-bottomed saucepan. Stir in the flour and cook, stirring constantly, until the paste cooks and bubbles a bit, but don't let it brown — about 2 minutes. Add the hot milk, continuing to stir as the sauce thickens. Bring it to a boil.Add into casserole bowl and mix all the ingredients together.Pour into an oven-safe casserole dish. Set aside.
Prepare the crust.Combine the crust ingredients in a small bowl.Sprinkle evenly over casserole.
Bake and serve.Bake at 350 degrees for approximately 45 minutes or until it starts to bubble.Remove from oven, serve, and enjoy.