01 - Crack the eggs into a mixing bowl. Add milk, salt, and pepper. Whisk vigorously until well combined and slightly frothy.
02 - Melt butter or heat olive oil in a non-stick skillet over medium heat, tilting to coat the surface evenly.
03 - Pour the egg mixture into the heated skillet and cook undisturbed for approximately 2 minutes, until the edges begin to set.
04 - Sprinkle shredded cheese evenly over the surface, followed by the diced tomato. Cover the skillet and cook for 2 to 3 minutes until the cheese starts to melt.
05 - Scatter the diced avocado (and chives or green onions, if using) onto one half of the omelette. Using a spatula, gently fold the opposite half over the filling.
06 - Cook the folded omelette for 1 additional minute, allowing the cheese to fully melt and the avocado to warm through.
07 - Slide the omelette onto a plate, slice in half, and serve immediately.