1. In a large skillet melt butter and saute thinly sliced white onions until browned, about 15 to 20 minutes. Remove the onions.
2. To the skillet add sirloin steak cut into 1/2-inch cubes. Brown well then return the onions to the pan.
3. In a small bowl combine flour, salt, and pepper. Add to the skillet and stir until blended. Gradually add beef broth and cook for 10 minutes.
4. Put fettuccine into casserole dish.
5. Pour contents of skillet over the cooked fettuccine and top with mozzarella cheese and Parmesan cheese. Broil until the cheese is melted and browned, about 5 minutes.