First, you'll want to get all your vegetables ready. Wash them thoroughly, then dice the bell peppers and tomatoes, and finely chop the onion. Chop the fresh spinach as well. Having everything prepped and set aside makes the cooking process much smoother and more enjoyable.

In a medium-sized bowl, crack the four large eggs. Add the milk, salt, and pepper. Whisk the mixture vigorously until it's light, airy, and slightly frothy. This step is important for achieving a fluffy and tender omelette texture.

Place a non-stick skillet over medium heat and add the olive oil or butter. Once it's hot, add the chopped onions and bell peppers. Sauté them for a few minutes until they soften and become fragrant. Next, add the diced tomatoes and chopped spinach, cooking for another two to three minutes until the spinach has wilted.

Pour the beaten egg mixture directly over the sautéed vegetables in the skillet. Let it cook for a minute or two without stirring. As the edges begin to set, use a spatula to gently lift them, allowing the uncooked egg from the center to flow underneath.

When the omelette is almost fully set but still a little moist on top, sprinkle the shredded cheddar cheese over one half of it. Carefully fold the other half of the omelette over the cheese. Let it cook for one more minute to ensure the cheese is completely melted and gooey.

Gently slide your finished omelette onto a plate. If you're using them, garnish with fresh parsley or chives for an extra burst of flavor and color. It's best served immediately while it's hot and the cheese is perfectly melted.