01 - Crack the eggs into a mixing bowl. Add milk, salt, and pepper. Whisk until well combined and slightly frothy.
02 - Heat butter or olive oil in a non-stick skillet over medium heat until the fat is fully melted and shimmering.
03 - Pour the egg mixture into the skillet and cook undisturbed for about 2 minutes, until the edges begin to set.
04 - Sprinkle shredded cheese evenly over the eggs, followed by the diced tomato. Cover the skillet and cook for 2 to 3 minutes until the cheese begins to melt.
05 - Add the diced avocado and chives or green onions (if using) on one half of the omelette.
06 - Using a spatula, gently fold the other half of the omelette over the filling. Cook for 1 more minute, allowing the cheese to finish melting and the avocado to warm through.
07 - Slide the omelette onto a plate, slice in half, and serve immediately.