Make the casserole. Spread the potatoes on the bottom of a 9 x 13 casserole dish.Note: If your mashed potatoes are cold, warm them first.Top with corn and cheese. Carefully slice the chicken strips into pieces of desired size and top the casserole dish with them. Bake uncovered for 15 minutes.
Make the gravy. Melt butter in a small saucepan over medium heat. Whisk in flour and stir until the mixture becomes paste-like and light golden brown, about 5 minutes. Whisk in beef broth gradually. Cook and stir until no lumps remain, about 2 minutes. Add Worcestershire sauce, Dijon mustard, onion powder, garlic powder, salt, and black pepper. Cook and whisk until gravy thickens, about 5 minutes more.