Whisk the egg and egg whites with a small pinch of salt and pepper until combined and slightly frothy.

Heat the olive oil in a non-stick pan over medium heat.

Add the spinach and tomato and cook for 1 minute, until the spinach wilts. Push them to one side.

Pour in the eggs. Let them set for about 30 seconds, then gently pull the edges toward the center, tilting the pan so uncooked egg runs underneath.

When the top is mostly set but still slightly glossy (2 to 3 minutes), sprinkle the feta over one half.

Fold the omelette in half, cook 30 seconds more, and slide onto a plate.