Crack eggs into a bowl, add salt and pepper, and whisk until well combined.

1. Heat olive oil or butter in a non-stick skillet over medium heat.
2. Add mushrooms and sauté for 3–4 minutes until browned and softened.
3. Add garlic and onion (if using), and cook for 1 minute until fragrant.
4. Add spinach and cook for 1–2 minutes until wilted. Remove mixture from pan and set aside.
5. Lightly grease the skillet again if needed. Pour in the beaten eggs and cook over medium-low heat.
6. When the eggs are nearly set but still slightly runny on top, spoon the spinach and mushroom filling over one half of the omelette.
7. Sprinkle cheese over the filling (if using), then fold the other half of the omelette over the filling.
8. Cook for 1 more minute until eggs are fully set and cheese is melted.
9. Slide onto a plate and serve immediately.