1. Preheat the broiler. Whisk the egg whites in a bowl until soft peaks form. Whisk the egg yolks in a bowl until pale and lightly thickened. Whisk in 2 tbsp water and season with salt and pepper. Add the whites to the yolks. Add 2 tbsp each of the Cheddar and Gruyére and fold together with a rubber spatula just until combined.

2. Heat the butter in an 7in (18cm) nonstick frying pan over medium heat until the butter is foaming. Add the egg mixture and spread evenly. Cook for about 1 minute, until the eggs look set around the edges. Run a heatproof spatula around the edge of the omelet to loosen it.

3. Sprinkle with the remaining Cheddar and Gruyére and the ham. Broil about 1 minute, until the top of the omelet is set. Run the spatula around the edge of omelet. Fold the omelet almost in half and slide onto the plate. Sprinkle with the chives and serve hot.