Preheat oven to 350 degrees Fahrenheit (175 degrees Celsius).

Line a 12-cup muffin tin with paper liners.

In a large bowl, combine finely chopped pepperoni, finely chopped green bell pepper, well-drained mushroom stems and pieces, finely chopped onion, grated Monterey Jack cheese, pizza sauce, garlic powder, and crushed red pepper flakes.

Mix all ingredients thoroughly to combine.

Incorporate slightly beaten eggs and cooking oil into the mixture.

Stir until well combined.

In a separate small bowl, whisk together all-purpose flour and baking powder.

Add the dry ingredients to the wet mixture.

Mix with a wooden spoon until a smooth batter forms.

Spoon the mixture evenly into the prepared muffin tins, filling each cup to the top.

Bake in the preheated oven for 22-25 minutes.

Check for doneness by inserting a toothpick into the center of a muffin; it should come out clean.

The muffins should be lightly browned.

Remove from the oven and let cool slightly.

Serve the pepperoni pizza muffins warm or cooled.

Store any leftover muffins wrapped in the refrigerator to maintain freshness.

Alternatively, freeze leftover muffins for longer storage.