Spread each toasted English muffin half with tomato paste, covering completely to edge.
Sprinkle with onion, Parmesan cheese, oregano, basil and cayenne pepper.
Arrange pepperoni on top and sprinkle with Cheddar cheese.
Line a dinner plate with paper towels.
Arrange 6 pizzas in ring on plate.
Microwave at high setting 1 1/2 minutes, or until cheese is melted, rotating plate one-quarter turn every 30 seconds.
Remove pizzas to serving platter.
Repeat with remaining pizzas.