Preheat the oven: Preheat the oven to 375°F (190°C).
Sauté the vegetables: In an oven-proof skillet, heat olive oil over medium heat. Add onion, garlic, and zucchini. Sauté until the zucchini is tender and lightly golden, about 8–10 minutes. If excess liquid forms, remove it with a spoon.
Whisk the eggs: In a medium bowl, whisk together eggs, milk, Parmesan, oregano, basil, salt, and pepper until well combined.
Combine and cook: Pour the egg mixture over the sautéed vegetables in the skillet. Cook over medium heat for 4–5 minutes, until the edges begin to set.
Bake the frittata: Transfer the skillet to the oven and bake for 10–15 minutes, or until the frittata is fully set and the top is golden.
Cool and serve: Allow to cool slightly before slicing. Serve warm, garnished with fresh basil or a drizzle of olive oil.