Heat oven to 400°F. In ungreased 1 1/2-quart casserole, mix onion, gravy mix, pepper, half-and-half and mustard. Gently stir in beef and mushrooms.
Place casserole on ungreased large cookie sheet. Place biscuits 1 inch apart on cookie sheet next to casserole.
Bake 17 to 23 minutes or until mixture is bubbly and thickened and biscuits are golden brown.
To serve, split biscuits in half; place bottom halves on individual serving plates. Top with beef mixture and top halves of biscuits.