Preheat your oven to 425°F.
In a large bowl, whisk together the eggs, milk, marinara sauce, oregano, and black pepper. Set aside.
Heat a 10-12-inch oven-safe skillet over medium heat and add the olive oil.
Once the oil is hot, add the diced onion and minced garlic, and sauté for about 4 minutes until the onions soften.
Add the spinach to the skillet and continue stirring until it wilts.
Pour the egg mixture into the skillet without stirring and let it cook for about 5-6 minutes until the edges begin to set.
Sprinkle the shredded mozzarella cheese over the top, followed by the pepperoni slices.
Transfer the skillet to the oven and bake for approximately 6 minutes until the frittata is fully cooked.
Remove from the oven, allow it to cool for a bit, then slice and serve! Enjoy your delicious pizza frittata!