Preheat the oven to 375°F (190°C).

In an oven-safe skillet, heat olive oil over medium heat.

Add diced onion and minced garlic, cooking until softened, about 3-4 minutes.

Stir in chopped spinach and cook until wilted, about 2 minutes.

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

Pour the egg mixture into the skillet over the sautéed vegetables. Gently stir to combine.

Sprinkle shredded cheese over the top of the egg mixture.

Cook on the stovetop for 2-3 minutes until the edges start to set.

Transfer the skillet to the oven and bake for 15-20 minutes, or until the center is set and lightly golden.

Let the frittata sit for a few minutes before slicing.