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

Chop the vegetables (bell pepper, onion, and spinach) and set aside.

Heat a non-stick skillet over medium heat and add the butter, allowing it to melt.

Add the chopped vegetables to the skillet and sauté for 2-3 minutes until softened.

Pour the egg mixture into the skillet, tilting the pan to ensure even coverage. Cook for 3-4 minutes until the edges begin to set.

Sprinkle the shredded cheddar cheese over one half of the omelette.

Using a spatula, carefully fold the omelette in half and cook for an additional 1-2 minutes until the cheese is melted and the eggs are fully cooked.

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