Preheat the oven to 375°F (190°C). In a large bowl, whisk together the eggs, milk, salt, and pepper until frothy. Set aside.

Heat the olive oil in a 10-inch oven-safe skillet over medium heat. Add the onion and cook until softened, about 5 minutes. Add the spinach in handfuls and stir until wilted, about 2 minutes. Spread the vegetables evenly in the skillet.

Pour the egg mixture over the vegetables. Scatter the feta and the optional Parmesan on top. Place the skillet over medium-low heat for 1 minute until the edges look opaque.

Transfer the skillet to the oven and bake for 20 to 25 minutes, until the frittata is puffed and the center is just set.

Let it rest for 5 minutes, then slide the frittata onto a cutting board. Slice into wedges and serve warm.