Pack mushrooms and vegetables into an omelette for a low calorie breakfast that fills you up on fiber and protein instead of fat and carbs.
Crack the eggs and separate the egg whites, whisking eggs and whites together until well combined and slightly frothy.
Heat half the olive oil in a nonstick skillet over medium-high heat; add mushrooms and sauté for 2 minutes, stirring occasionally.
Add onion and bell pepper to the pan, reduce heat to medium, and cook for another 2-3 minutes until vegetables begin to soften.
Toss in the baby spinach and cook, stirring gently, until just wilted; season the vegetables with salt and pepper.
Reduce the heat to medium-low, push the vegetables to one side, add remaining olive oil, and pour in the egg mixture; let set without stirring for 60 seconds.
Using a spatula, gently lift the set edges and tilt the pan to allow uncooked egg to flow to the sides; when just set but still moist on top, spoon the veggies evenly over half the omelette.
Fold the omelette over the filling, cover the skillet, and cook on low for an additional 60 seconds for a fluffy center; slide onto a plate and serve immediately.