Preheat your oven to 400°F (200°C) and line a rimmed sheet pan with parchment paper to ensure easy removal.
Dice the red bell pepper, zucchini, and red onion into uniform half-inch pieces to ensure even cooking.
Place the vegetables on the prepared sheet pan, drizzle with olive oil, and sprinkle with sea salt, black pepper, and dried oregano.
Toss the vegetables thoroughly to coat and roast in the oven for 12-15 minutes until they are tender and slightly caramelized at the edges.
While the vegetables roast, whisk the whole eggs and liquid egg whites together in a medium bowl until the mixture is frothy and well combined.
Remove the sheet pan from the oven and use a spatula to spread the roasted vegetables into an even layer across the pan.
Carefully pour the egg mixture over the vegetables, ensuring it reaches all corners of the pan, then sprinkle the crumbled feta cheese evenly over the top.
Return the pan to the oven and bake for an additional 8-10 minutes, or until the eggs are fully set and slightly puffy.
Let the sheet pan cool for 2 minutes before slicing into large squares and serving warm.