Preheat your oven to 375°F (190°C) and line a rimmed 13x18-inch sheet pan with parchment paper for easy cleanup.
Dice the bell peppers and red onion into small, uniform pieces. Place them on the sheet pan, drizzle with olive oil, and toss to coat.
Roast the vegetables in the oven for 10 minutes until they are slightly softened and fragrant.
While the vegetables roast, whisk together the whole eggs, liquid egg whites, sea salt, black pepper, and garlic powder in a large bowl until the mixture is uniform and slightly frothy.
Remove the sheet pan from the oven. Roughly chop the baby spinach and scatter it evenly over the roasted peppers and onions.
Carefully pour the egg mixture over the vegetables on the sheet pan, ensuring it reaches all the corners. Sprinkle the crumbled feta cheese evenly across the top.
Return the pan to the oven and bake for 12-15 minutes, or until the eggs are completely set in the center and the edges are beginning to turn golden.
Let the eggs cool for a few minutes before slicing into squares. Serve warm or store for a quick grab-and-go breakfast during the week.