Position an oven rack in the center of the oven and heat the oven to 400°F (200°C). In a medium bowl, whisk the eggs, milk, salt, pepper, Italian seasoning, 3/4 cup mozzarella, and all of the Parmesan until combined.

Heat the olive oil in a 10-inch oven-safe skillet over medium heat. Add the onion and cook, stirring occasionally, until softened. Add the garlic and stir just until fragrant.

Add the spinach by the handful, stirring until it is fully wilted and any excess moisture has cooked off. Spread the vegetables evenly across the skillet.

Pour the egg mixture over the spinach. Tilt the skillet gently so the eggs settle around the vegetables. Cook without stirring until the edges begin to set.

Sprinkle the remaining 1/4 cup mozzarella over the top. Transfer the skillet carefully to the oven and bake until the center is set and no longer looks wet when the skillet is gently shaken. A thermometer inserted in the center should read 160°F (71°C).

Remove the skillet from the oven using an oven mitt and let the frittata stand briefly before slicing into wedges. Serve warm or at room temperature.