Preheat your oven to 450°F (232°C).

In a medium bowl, whisk together the eggs, milk, sea salt, and black pepper until fully combined.

Heat olive oil in a skillet over medium heat, then sauté the diced potatoes for about 5 minutes until fork-tender.

Add the diced red pepper and green onion; cook for an additional 3-4 minutes.

Stir in the chopped spinach and cook until wilted.

Remove from heat and pour the egg mixture over the vegetables; gently stir and top with crumbled feta.

Allow the frittata to set on the stovetop for 2-3 minutes.

Transfer to the oven and bake for 15 minutes.

Once baked, allow to cool slightly, then slice into wedges for serving.