1. Set oven to 220 degrees (fan).
2. Halve the peaches, removing the stones, then place on the baking sheet, flat side up.
3. Drizzle the honey over the peach halves, then sprinkle the sugar, trying to distribute both evenly.
4. Remove the buds from the lavender stems and sprinkle across the fruit.
5. Cover the baking sheet with foil and bake in the oven for 15 minutes.
6. Remove the foil, add the blackberries, then bake uncovered for a further 7-10 or until the peaches are soft and the blackberries still hold their shape.