In a mixing bowl, crack the four eggs and whisk them together until well combined. Add a pinch of salt and pepper to taste.
Heat a non-stick frying pan over medium heat and add olive oil (or butter). Allow it to heat up.
Add the chopped onion to the pan and sauté until it becomes translucent (about 2-3 minutes).
Stir in the chopped spinach and cook for an additional 1-2 minutes until the spinach is wilted.
Pour the beaten eggs over the sautéed vegetables, tilting the pan to spread the eggs evenly.
Sprinkle the shredded cheese on top of the eggs.
Cook for about 3-4 minutes until the edges start to set. Use a spatula to gently lift the edges and let any uncooked egg flow underneath.
Once the omelette is mostly set but still slightly runny on top, carefully fold it in half using the spatula.
Let it cook for another minute, then slide the omelette onto a plate and serve hot.
For added flavor, consider customizing your omelette with extra ingredients like tomatoes, bell peppers, or mushrooms.