Heat oven to 400°F. Grease two 10-oz custard cups or 12-oz (1 1/2-cup) casseroles.
In 8-inch skillet, cook beef and onion over medium-high heat 5 to 7 minutes, stirring frequently, until beef is thoroughly cooked; drain.
Stir in pizza sauce and mushrooms; heat to boiling, stirring occasionally. Reduce heat; simmer uncovered 5 minutes, stirring occasionally. Spoon mixture into casseroles. Sprinkle 1/4 cup mozzarella cheese on each. Top each with frozen biscuit; sprinkle with Parmesan cheese. Place custard cups on cookie sheet with sides.
Bake 18 to 22 minutes or until biscuits are deep golden brown.