Preheat the oven to 425°F. Place a medium oven-safe skillet over medium heat and add the olive oil. When the oil is shimmering, stir in the diced onion and minced garlic. Cook, stirring occasionally, until the onion softens and turns translucent and the garlic smells fragrant, about 5 minutes. Add the baby spinach to the skillet and cook for another 2 to 3 minutes, tossing gently, until the leaves are fully wilted. Season the vegetables lightly with salt.

In a large mixing bowl, crack in the eggs and whisk until the yolks and whites are fully combined. Add the dried oregano, skim milk, and marinara sauce, whisking again until the mixture is smooth and evenly colored. Season with a pinch of salt and a few grinds of black pepper, keeping in mind that the cheese and pepperoni will also add seasoning.

Pour the egg and marinara mixture evenly over the sautéed onions, garlic, and spinach in the skillet, gently nudging the vegetables so they are distributed throughout. Let the frittata cook on the stovetop over medium heat until the edges begin to set and pull slightly away from the sides of the pan, about 4 to 5 minutes, while the center remains somewhat loose.

Scatter the shredded mozzarella evenly over the surface, then arrange the sliced pepperoni on top, spacing them so each slice of frittata will get a bit of “pizza” topping. Transfer the skillet carefully to the preheated oven and bake for 5 to 6 minutes more, or until the eggs are fully set in the center and the cheese is melted and bubbling.

Remove the skillet from the oven and let the pizza frittata rest for a couple of minutes before slicing into wedges. Serve warm, straight from the pan, for a dish that brings together the best parts of pizza night and a hearty egg breakfast.