Preheat your oven to 375°F and lightly grease a 9x13 inch rimmed sheet pan with the olive oil.
Spread the diced bell peppers and red onions across the pan in a single layer and roast for 10 minutes until they begin to soften.
While the vegetables roast, whisk together the whole eggs, egg whites, sea salt, black pepper, and garlic powder in a medium bowl until well combined.
Remove the sheet pan from the oven and scatter the baby spinach over the roasted vegetables, stirring slightly to let the heat wilt the leaves.
Pour the egg mixture evenly over the vegetables, ensuring it reaches all corners of the pan.
Sprinkle the crumbled feta cheese across the top of the egg mixture.
Return the pan to the oven and bake for 12-15 minutes, or until the eggs are set and no longer jiggle in the center.
Allow the eggs to cool for 2 minutes before slicing into squares and serving.