Heat olive oil in a non-stick skillet over medium heat and sauté mushrooms for 3-4 minutes until browned.

Add spinach to the skillet, cook until wilted, then remove vegetables and set aside.

Whisk together eggs, egg whites, salt, and pepper in a bowl.

Pour egg mixture into the skillet over medium-low heat and cook undisturbed for 2 minutes.

Lift the edges of the omelet to let uncooked egg flow underneath and cook for an additional 1-2 minutes.

Add the mushroom and spinach mixture and cheese to one half of the omelet.

Fold the omelet over the filling, cook for 1 minute until cheese is melted, and serve.