Heat the oven to 450°F.
Place the English muffins on a lined sheet pan cut side up. Spoon on a little tomato sauce. Use the back of the spoon to spread the tomato sauce to cover the muffin.
Sprinkle some mozzarella cheese on each muffin. Add 4 slices each to 6 muffin halves.
Add some bell pepper, mushrooms, and red onions to the other 6 halves.