Preheat your oven to 375°F and ensure a rack is positioned in the center.
In a medium-sized bowl, whisk together the eggs, egg whites, sea salt, black pepper, and garlic powder until the mixture is well combined and slightly frothy.
Heat the extra virgin olive oil in a 10-inch oven-safe skillet over medium heat, swirling to coat the bottom.
Add the sliced shiitake mushrooms to the skillet and sauté for 5 to 6 minutes until they are tender and golden brown.
Stir in the baby spinach and cook for 1 to 2 minutes until just wilted, then spread the vegetables into an even layer.
Pour the egg mixture over the sautéed vegetables in the skillet, then sprinkle the crumbled feta cheese evenly across the surface.
Allow the frittata to cook on the stovetop for about 2 minutes until the edges are set and pull away slightly from the pan.
Transfer the skillet to the oven and bake for 12 to 15 minutes, or until the center is firm to the touch and the top is lightly golden.
Remove from the oven and let the frittata rest for 5 minutes before slicing into wedges and serving warm.