Crack eggs into a bowl, whisk until smooth and frothy. Slice avocado into thin pieces.
Heat butter in a non-stick pan over medium-low heat until foamy.
Pour in eggs, let sit 30 sec. Push edges inward with spatula, tilt pan to let uncooked egg flow.
Place avocado slices on one half of omelette. Add cheese, spinach, or tomato if using.
Cook 1 min to warm fillings.
Fold omelette in half with spatula, covering filling.
Slide omelette onto a plate. Garnish with chives or paprika before serving.
Finished omelette on plate