Baked Ziti I

INGREDIENTS

• 1 pound dry ziti pasta
• 1 onion, chopped
• 1 pound lean ground beef
• 2 (26 ounce) jars spaghetti sauce
• 6 ounces provolone cheese, sliced
• 1 1/2 cups sour cream
• 6 ounces mozzarella cheese, shredded
• 2 tablespoons grated Parmesan cheese