Preheat your oven to 190°C (375°F) and lightly grease a baking dish with olive oil

In a large bowl, whisk together the eggs, whole milk, sea salt, and black pepper until well combined and slightly frothy

Heat the olive oil in a skillet over medium heat, then add the sliced red onion and cook until softened, about 3–4 minutes

Stir in the minced garlic and cook until fragrant, about 30 seconds

Add the chopped baby spinach to the skillet and sauté until wilted, about 2–3 minutes

Remove the skillet from heat and transfer the sautéed vegetables into the prepared baking dish

Scatter the halved cherry tomatoes, crumbled feta cheese, chopped basil, and oregano evenly over the vegetables

Pour the egg and milk mixture over the ingredients in the baking dish, gently shaking the pan to distribute evenly

If using, sprinkle Kalamata olives and red pepper flakes on top for extra flair

Place the dish in the preheated oven and bake for 20–25 minutes, or until the edges are golden and the center is set

Remove from the oven and let the frittata cool for a few minutes before slicing and serving