In a medium bowl, Whisk egg yolks and milk. Gently fold the egg whites into the egg yolk mixture.
Heat a nonstick pan over medium heat and add the butter. Pour the egg mixture into pan and cook until the underside is golden. Gently flip the omelette over, using spatula to ease it over. Cook for another few seconds.
Place avocado over half of omelette, cover it with cheese slices and add tomatoes. Fold the other edge of the omelette over the filling. Serve immediately.