Whisk eggs, egg whites and onion in small jug.

Spray small frying pan with cooking oil, heat pan. Pour half of the egg mixture into pan, cook, tilting pan, until mixture is almost set. Sprinkle half of the spinach and mint over half the omelette, fold omelette over to enclose filling, then cut in half. Repeat with remaining egg mixture, spinach and mint.

Spread toast with cheese, serve with omelette.