Melt butter in a saucepan over medium heat. Whisk in flour and cook for 2 minutes. Gradually whisk in chicken stock. Reduce heat to low and cook and stir until thick enough to coat the back of a spoon, 4-6 minutes. Stir in heavy cream, and season with salt, pepper, and cayenne pepper.

Serve the casserole with gravy over the top and a big side of mashed potatoes.