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 and cook for 1-2 minutes until wilted, then remove vegetables and set aside.

Whisk together eggs, egg whites, salt, and black 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 allow uncooked egg to flow underneath and cook for another 1-2 minutes.

Add the sautéed mushrooms, spinach, and cheese to one half of the omelet.

Fold the omelet over the filling and cook for 1 additional minute until the cheese melts.