In a mixing bowl, whisk together the eggs, milk, salt, and pepper until well combined.

Heat the non-stick skillet over medium heat and add the butter. Once melted, add the diced onions and bell peppers. Sauté for about 3-4 minutes until softened.

Add the spinach and cook for an additional 1-2 minutes until wilted.

Pour the egg mixture over the cooked vegetables in the skillet. Cook for about 5-7 minutes, gently lifting the edges with a spatula to allow uncooked eggs to flow underneath.

Once the eggs are set but still slightly runny on top, sprinkle the shredded cheese over half of the omelette.

Fold the omelette in half and cook for another minute until the cheese is melted.

Carefully slide the omelette onto a plate, cut into wedges, and serve warm.