Yes, the rhubarb needs to be pre-cooked in this recipe. All elements are baked or cooked separately and the rhubarb will not be cooking any further in the oven. For this recipe, I made a stewed rhubarb filling so I pushed the cooking until the fruit started to fall apart. But you could also reduce the cooking time and simply poach the fruits for a few minutes or until soft and create a fun graphic pattern with them.