To make this pizza frittata, start by preheating your oven to 425 F.
Next, crack the eggs into a large bowl, and add in the milk, marinara sauce, oregano, and black pepper. Whisk until the ingredients are fully combined, then set the bowl aside.
Next, heat a 10-12-inch oven-safe skillet over medium heat and add in the olive oil or avocado oil (if using).
While the oil is heating, dice the onion and mash the garlic.
Then, once the oil is hot, add in the onion and garlic and sauté them for approximately 4 minutes, until the onions begin to soften.
Once the onions are soft, add in the spinach and stir until it has wilted.
Next, pour the egg mixture into the pan and allow it to cook, undisturbed. This is important! Allow it to sit for 5-6 minutes, until the edges have begun to set.
Then, sprinkle the mozzarella cheese on top of the egg mixture, followed by the pepperoni slices.
Transfer the frittata to the oven and allow it to cook for 6 minutes, until the eggs are cooked through.
Finally, once it's done, remove it from the oven and allow it to cool slightly before slicing. Slice the frittata, then serve and enjoy! Store any leftovers in an airtight container, in the fridge, for up to one week, or in the freezer for up to one month.