Preheat oven to 450 °F. In a medium bowl, whisk together the eggs, milk, salt and pepper. Set aside.

In a large oven-safe skillet, heat olive oil over medium heat. Add the diced potatoes and sauté for at least 5 minutes, stirring occasionally. Season with salt and pepper.

Add the red pepper and green onion and continue to sauté for another few minutes, until they start to soften. Stir in the chopped spinach and cook until wilted.

Turn off the heat and add the egg mixture evenly over top of the vegetables, then top with the crumbled feta cheese. Leave the skillet on the hot burner for 2-3 minutes, until the eggs start to set around the edges.

Carefully transfer the skillet to the oven and bake for up to 15 minutes, or until the eggs are set in the middle of the frittata. Serve immediately, or if making it for meal prep, cut it into slices and store in individual sealed storage containers in the refrigerator. (Be sure not to touch the skillet handle with your bare hands after removing it from the oven!). Eat within 4 days.