Preheat your oven to 375°F (190°C). Ensure you have a 9-inch oven-safe non-stick skillet ready.

Heat the olive oil in the 9-inch oven-safe non-stick skillet over medium heat. Add the finely chopped shallot and cook for 3-4 minutes, stirring occasionally, until it has softened and become translucent.

Add the minced garlic to the skillet with the shallots. Cook for another minute until fragrant, being careful not to burn the garlic.

Add the roughly chopped fresh spinach to the skillet. Cook, stirring frequently, for 3-5 minutes until the spinach has completely wilted and reduced significantly in volume. Remove the skillet from the heat.

In a large mixing bowl, whisk together the large eggs, milk, salt, and black pepper until the mixture is well combined and slightly frothy.

Gently fold the crumbled feta cheese into the egg mixture. Pour the egg and feta mixture evenly over the wilted spinach and shallot/garlic in the skillet. Use a spoon to evenly distribute the spinach and feta if needed.

Place the skillet back on the stovetop over medium-low heat. Cook for 5-7 minutes, until the edges of the frittata begin to set and pull away from the sides of the pan, and the bottom is lightly golden.

Carefully transfer the skillet to the preheated oven. Bake for 15-20 minutes, or until the frittata is puffed, golden brown on top, and set in the center. To check for doneness, a knife inserted into the center should come out clean.

Remove the frittata from the oven and let it rest in the skillet for 5 minutes before slicing it into wedges and serving. Enjoy warm or at room temperature.