For the sabayon, put the egg yolks, water, sugar and elderflower liqueur into a bowl and place over a pan of boiling water. Whisk constantly for 6-8 minutes until the mixture becomes light in colour and consistency. It should be a thick pouring consistency. You may need to let it down with some water. Remove from the heat. Cover with cling film and keep warm until required.
To make the lavender shortbread, cream the butter and sugar together, beat the egg yolks in and then sift in the flour. Mix by hand to a soft dough, but don’t overwork the paste. Add the fresh lavender petals and essence. Roll into a sausage and sprinkle the outside with granulated sugar. Wrap in cling film and set in the fridge.
Remove from the fridge and slice about 1cm thick, place on a non-stick baking tray, and then
Place your summer berries into a dish and pour the elderflower mix over the top, place under a hot grill and cook until golden brown. You can use a blowtorch if you wish.
Finish the gratin with sprigs of fresh mint and lavender flowers. Serve with the shortbread.