Add olive oil to a nonstick skillet over medium heat. Add onions, bell peppers, and mushrooms. Sauté for about 3-4 minutes until the vegetables are softened.
Add spinach and cook until just wilted, about 1 minute. Remove from heat and set aside.
In a bowl, whisk the eggs with salt and pepper until well combined.
Pour the beaten eggs into the skillet over medium heat. Cook for 1-2 minutes until the edges start to lift from the pan. Spoon the sautéed vegetables over half of the omelette and sprinkle with cheddar cheese.
Fold the omelette over the filling and cook until the cheese is melted. Carefully slide the omelette onto a plate and serve hot with fresh parsley sprinkled on top.