1. For the fruit: Preheat the oven to 350°F. Butter a shallow 3-quart baking dish. Spread the rhubarb in the dish.
2. Pour the sugar and tapioca over the strawberries; stir well and let the mixture sit for at least 15 minutes, so the juice from the berries will soften the tapioca. After this rest, pour the berries and sugar over the rhubarb in the baking dish.
3. For the topping: Whisk together the flour, sugar, baking powder and salt.
4. Add the milk, egg, and vanilla to the melted butter; whisk the mixture together and pour into the dry ingredients.
5. Stir the batter together until it's evenly moistened.
6. Spread the batter over the fruit in the pan, and sprinkle it with sparkling sugar.
7. Bake for 40 to 45 minutes, until the fruit bubbles at the edges and the top is golden brown. Remove from the oven and serve warm.