Heat the oven to 180C/fan 160C/gas 4. Choose a baking dish or roasting tin that will hold the plum halves snugly in a single layer. Rub the butter all over the base, sprinkle the 2 tbsp of sugar on top, then scatter over all but one of the vanilla pod pieces. Arrange the plums over the top, cut-side down, and drizzle over 2 tbsp of water. Roast for 10-15 minutes or until the plums are really tender.
Meanwhile, combine the remaining 75g of sugar, remaining vanilla pod piece, 150g of the blueberries and the balsamic vinegar in a pan. Heat and bubble fiercely for 8-10 minutes or until the blueberries are breaking down, and a syrupy sauce has formed. Tip in the remaining blueberries then take off the heat.
When the plums are cooked, flip them over and pour the blueberries all over them. Leave for at least 10 minutes, then eat warm or at room temperature with scoops of ice cream, if you like.
*This recipe is gluten-free according to industry standards