Preheat your oven to 375°F (190°C).
In a medium bowl, whisk together the large eggs, liquid egg whites, sea salt, and black pepper until well combined and slightly frothy.
Heat the extra virgin olive oil in a 10-inch oven-safe non-stick skillet over medium heat.
Add the diced red onion and sauté for 3-4 minutes until translucent and soft.
Stir in the baby spinach and halved cherry tomatoes, cooking for 1-2 minutes until the spinach is just wilted.
Pour the egg mixture into the skillet over the vegetables, ensuring they are evenly distributed.
Sprinkle the crumbled feta cheese, fresh basil, and fresh parsley over the top of the eggs.
Allow the frittata to cook undisturbed on the stovetop for 2-3 minutes until the edges begin to set.
Transfer the skillet to the preheated oven and bake for 10-12 minutes, or until the center is set and the top is lightly golden.
Remove from the oven, let it cool for a few minutes, then slice into wedges and serve.