"Chicago Style" Pizza

Ingredients

• 1 1/2 lb. ground beef
• 1/2 c. chopped onion
• 2 Tbsp. grated Parmesan cheese
• 1 to 1 1/2 tsp. oregano
• 1 1/4 tsp. salt
• 1/2 tsp. pepper
• 1 small can tomato paste
• 1 (10 biscuit) can refrigerator buttermilk or country-style biscuits
• 1 (No. 202) can (1/4 c.) mushroom stems and pieces, drained
• 1 tomato, sliced
• 1 c. (4 oz.) shredded Mozzarella cheese