Heat the oven to 375°F (190°C). Whisk the eggs, milk, Parmesan, salt and pepper in a large bowl.
Heat 2 tablespoons of the oil in a 10-inch ovenproof nonstick or well-seasoned cast-iron skillet over medium heat. Add the potato slices and onion with a pinch of salt. Cover and cook, turning occasionally, until the potatoes are tender and lightly golden, 10–12 minutes.
Add the garlic and spinach and stir until the spinach wilts, 1–2 minutes. Spread the vegetables evenly and drizzle in the remaining oil around the edges.
Pour in the eggs and gently shake the pan so they settle around the vegetables. Scatter the feta on top. Cook without stirring for 3–4 minutes, until the edges begin to set.
Transfer the skillet to the oven and bake 12–15 minutes, until the center is just set and no longer jiggles.
Let cool for 5 minutes, then run a spatula around the edges and slide onto a board. Sprinkle with herbs and cut into wedges.