Preheat your oven to 400°F (200°C) and line a small rimmed sheet pan with parchment paper.
Dice the red bell pepper, green bell pepper, and yellow onion into small, uniform pieces.
Place the diced vegetables on the sheet pan and toss with olive oil, sea salt, black pepper, and dried oregano.
Roast the vegetables for 10-12 minutes until they are tender and slightly charred around the edges.
While the vegetables roast, whisk together the whole eggs and liquid egg whites in a medium bowl until fully combined.
Remove the pan from the oven, spread the roasted vegetables into an even layer, and pour the egg mixture over them.
Sprinkle the crumbled feta cheese evenly across the top of the eggs.
Return the sheet pan to the oven and bake for another 10-12 minutes, or until the eggs are completely set and slightly golden on top.
Allow the eggs to cool for a few minutes before slicing into squares and serving.