Preheat a small ovenproof frying pan in the oven to 200C/400F/Gas 6.
For the filling, place the blueberries and sugar into a small saucepan and cook gently for 3-4 minutes.
For the omelette, place the egg whites into a large bowl and whisk until stiff.
Place the egg yolks and sugar into a separate bowl and whisk until frothy.
Pour the egg whites into the yolk mixture and fold in gently.
Remove the frying pan from the oven and place the butter and oil into the pan to melt, then pour the egg mixture into the pan.
Pour the blueberry mixture onto the middle of the egg mixture, transfer to the oven and bake for 4-5 minutes, or until set.
To serve, remove the pan from the oven and dust with icing sugar.