Coat inside of Cooking Pot with nonstick cooking spray. Combine beef, sausage, bread crumbs, 3 cups Parmesan cheese and egg in large bowl; mix well. Shape beef mixture into 64 (1½-inch) meatballs. Place meatballs in Cooking Pot. Pour pasta sauce over meatballs. Secure lid. Press MEAT/POULTRY, set pressure to HIGH and time to 15 minutes. Make sure Steam Release Valve is in "Seal" (closed) position. Press START/STOP.
Once cooking is complete, quick release pressure. If desired, remove meatballs to medium bowl using slotted spoon; cover to keep warm. Press BROWN/SEAR. Cook sauce, uncovered, on HIGH 5 to 10 minutes or until thickened. Return meatballs to Cooking Pot; toss to coat. Season with red pepper flakes, if desired. Serve meatballs and sauce in rolls; top with additional cheese.