Preheat your oven to 400°F and line a rimmed sheet pan with parchment paper for easy cleanup.
Dice the red bell pepper and red onion into small, uniform pieces to ensure even roasting.
Place the diced vegetables on the sheet pan, drizzle with olive oil, sprinkle with half of the salt and pepper, and toss to coat thoroughly.
Roast the vegetables in the oven for 12 minutes until they are tender and just beginning to brown at the edges.
While the vegetables roast, whisk the whole eggs, liquid egg whites, garlic powder, and the remaining salt and pepper in a medium bowl until well combined.
Remove the pan from the oven and scatter the baby spinach over the hot vegetables, tossing slightly until the leaves begin to wilt.
Spread the vegetable mixture into an even layer across the pan, then carefully pour the egg mixture over the top.
Sprinkle the crumbled feta cheese evenly over the eggs and return the pan to the oven.
Bake for 8 to 10 minutes until the eggs are set and slightly golden, then slice into squares and serve hot.