Heat olive oil in a large non-stick skillet over medium-high heat.
Add the sliced cremini mushrooms and diced yellow onions to the pan.
Sauté for five minutes until the vegetables are golden brown and soft.
Season with 1/4 teaspoon salt and a pinch of pepper.
Stir in the baby spinach until wilted, then remove from the pan and set aside.
Wipe the skillet clean and return to medium-low heat with the butter.
Whisk egg whites with remaining salt and pepper in a medium bowl.
Pour egg whites into the skillet and cook until edges begin to set.
Add the vegetable filling and shredded cheese to one half of the omelette.
Fold the omelette over the filling and cook for another minute until the cheese is melted.
Slide onto a plate and garnish with fresh parsley to serve.