Pre-heating & Aromatic Foundation: Position an oven rack in the center and preheat to 400°F (200°C). Heat the extra virgin olive oil in your 10-inch oven-safe skillet over medium heat. Add the diced onion and red bell pepper, cooking for 4-5 minutes until softened and translucent. Stir in the minced garlic and cook for another minute until fragrant.
Wilting the Verdant Base: Add the fresh baby spinach to the skillet in large handfuls. Stir continuously until all the spinach has wilted down completely, about 2-3 minutes. Remove the skillet from the heat and set it aside.
Creating the Egg Emulsion: In a large mixing bowl, combine the 5 whole eggs and 1 cup of egg whites. Add the unsweetened almond milk and whisk vigorously for 1 minute until the mixture is frothy, pale, and uniform. This incorporates air for a lighter texture.
Flavor Infusion: To the egg emulsion, add the chopped parsley, mint, dill, dried oregano, black pepper, red pepper flakes, and lemon zest. Whisk gently to combine all the herbs and spices.
Component Integration: Pour the seasoned egg mixture directly into the skillet over the cooked vegetables. Use a spatula to gently distribute the spinach and aromatics evenly throughout the eggs.
Stovetop Setting & Oven Finalization: Return the skillet to medium heat on the stovetop. Cook, without stirring, for 3-4 minutes, just until the edges of the frittata begin to set. Transfer the skillet to the preheated oven and bake for 8-10 minutes, or until the center is just set and does not jiggle when the pan is gently shaken.
Garnish, Rest, and Serve: Carefully remove the skillet from the oven. Immediately sprinkle the crumbled feta cheese evenly over the hot surface. Let the frittata rest in the pan for 5 minutes; this allows it to set fully and makes slicing easier. Slice into 6 wedges and serve warm.