Clean the fruit. Rhubarb, gooseberries or apples can be used instead of blackberries. Roll out the pastry into a circle and put the fruit in half of it. Fold over the pastry and seal the edges. Bake at 375°F, 190°C, gas mark 5 for forty minutes. Take from the oven and just before serving, gently lift the top and insert small pieces of butter and sprinkle in the brown sugar.
This is a Welsh dish which used to be cooked on a warm greased bakestone, and turned half-way through cooking. It is easier, now, to bake the tart, but it can be cooked in a thick frying pan instead.