Crack the eggs into a bowl and whisk until well beaten.
Heat the olive oil in a non-stick pan over medium heat.
Add the onions, bell peppers, and mushrooms to the pan and sauté for a couple of minutes until they soften.
Pour the beaten eggs into the pan and spread them out evenly with a spatula.
Sprinkle the shredded cheese on the eggs and season with salt and pepper to taste.
Cook for 2-3 minutes, or until the edges of the omelette start to firm up and the bottom is lightly golden.
A spatula folds the omelette in half and slides it onto a plate.
Serve immediately and enjoy!