Warm the sliced strawberries, redcurrant jelly and vinegar together in a saucepan until the jelly has just melted.
Meanwhile, whisk the egg whites into stiff, moist-looking peaks. Mix the egg yolks with 1 tablespoon of the sugar, then fold into the egg whites.
Heat the butter in a large frying pan, add the egg mixture and cook over a medium heat for 3—4 minutes until the underside is golden. Quickly transfer the pan to a hot grill and cook for 2—3 minutes until the top is browned and the centre still slightly soft, making sure that the handle is away from the heat.
Spoon the warm strawberry mixture over the omelette, fold in half and dust with the remaining sugar. Cut into 4 and serve immediately with extra strawberries.