Mix catsup, oregano, vinegar, onion powder, sugar and pepper.
Spread one-fourth on each muffin half.
Top each with one-fourth of the pepperoni.
Place 1 tablespoon mozzarella on each muffin half; sprinkle with Parmesan.
Place on single layer of paper towels on turntable or on double layer of paper towels on dinner plate.
Microwave at HIGH (100%) until cheese is melted and filling is heated through, 45 seconds to 1 minute 45 seconds.
Cut each muffin half into 4 wedges.