Preheat your oven to 400°F and line a small rimmed sheet pan with parchment paper to prevent sticking.
Place the diced bell peppers, zucchini, and red onion on the sheet pan and drizzle with the olive oil.
Season the vegetables with sea salt, black pepper, garlic powder, and dried oregano, then toss to coat evenly.
Roast the vegetables for 12 to 15 minutes until they are tender and just beginning to brown at the edges.
Remove the pan from the oven and stir in the fresh baby spinach, allowing the residual heat to wilt the leaves slightly.
In a medium bowl, whisk together the whole eggs and the liquid egg whites until the mixture is uniform and frothy.
Pour the egg mixture carefully over the roasted vegetables, tilting the pan if necessary to ensure it spreads to all corners.
Sprinkle the crumbled feta cheese evenly over the top of the eggs.
Return the sheet pan to the oven and bake for an additional 8 to 10 minutes, or until the eggs are completely set and firm to the touch.
Let the pan cool for 2 minutes before slicing the egg bake into squares and serving.