Preheat the oven to 350 degrees. Prepare two muffin tins with cupcake liners. Spritz the liners lightly with nonstick cooking spray.
In a medium skillet over medium-high heat, add the olive oil, onion, and bell pepper. Cook until softened, about 5 minutes, stirring as needed.
In a large bowl, mix all the ingredients, including the onion and bell pepper, until combined.
Using a large cookie scoop, fill the muffin cups to about three-fourths full.
Bake for 17 to 20 minutes.
Transfer muffin tins to a wire rack and cool for 5 minutes before removing muffins from the baking pan.
Serve with additional warm pizza sauce if desired.