Preheat your oven to 400°F (200°C) and line a large rimmed sheet pan with parchment paper for easy cleanup.
Place the diced red bell pepper and red onion on the sheet pan, drizzle with olive oil, and sprinkle with sea salt, black pepper, and garlic powder.
Toss the vegetables to coat evenly and spread them out in a single layer.
Roast the vegetables in the oven for 10 to 12 minutes until they are tender and just beginning to brown.
While the vegetables roast, whisk the whole eggs and liquid egg whites together in a medium bowl until the mixture is uniform and frothy.
Remove the pan from the oven, add the chopped baby spinach, and toss briefly with the hot vegetables until the leaves begin to wilt.
Ensure the vegetable mixture is spread evenly across the pan, then carefully pour the egg mixture over the top, tilting the pan if necessary to cover the entire surface.
Return the pan to the oven and bake for 8 to 10 minutes, or until the eggs are completely set in the center and slightly puffed.
Remove from the oven, let cool for 2 minutes, then slice into large squares and serve warm.