Preheat the oven to 400 degrees F. Line a regular-sized muffin tin with silicon liners, or if you don’t have them, spray the wells of the tin with the nonstick cooking spray. Set aside.
In a large bowl, combine the flour, baking powder, salt, garlic, black pepper, red pepper flakes, and oregano. Use a whisk to mix the ingredients together thoroughly.
In a separate large bowl, add the egg and beat lightly. Add the milk, the marinara sauce, the mozzarella and feta cheese, then fold in the olives, bell pepper, and green onion. Mix to combine.
Add the dry mixture to the wet mixture, a little at a time. Mix until the ingredients are combined, but take care not to overmix.
Spoon the batter evenly into each muffin liner (or well of the tin), 3/4 of the way to the top.
Bake in the middle rack for 15-18 minutes, or until the tops are lightly golden and a toothpick inserted into the center of a muffin comes out clean.
I added a tiny bit of mozzarella cheese to the tops of each muffin when I took them out of the oven.
Cool on a wire rack.
Serve warm or at room temperature, and with extra marinara sauce for dipping (optional).