Preheat your oven to 375°F (190°C).

In a large skillet over medium heat, add a splash of olive oil. Sauté the diced onion until translucent (3-4 minutes), then add minced garlic and spinach until wilted (about 1 minute).

In a mixing bowl, whisk together eggs, milk, salt, and pepper until combined.

Pour the egg mixture over the sautéed vegetables in the skillet. Gently stir to combine and sprinkle cheese on top.

Transfer the skillet to the oven and bake for 20-25 minutes or until the frittata is set and golden brown on top.

Let cool for a few minutes before slicing into wedges. Serve warm or at room temperature.