Preheat oven to 350°F.
Add all the berry filling ingredients—berries, sugar, thickener of choice, lemon juice, and salt—straight into a greased 9×13-inch baking dish and stir to combine.
If using instant tapioca, stir until evenly distributed. If using tapioca flour, stir very well so the flour fully dissolves and coats the berries (no dry pockets).
In a medium bowl, whisk the flour(s), sugar, baking powder, and salt. Add eggs and mix with a fork until crumbly.
Sprinkle the topping evenly over the berries, then drizzle with the melted butter or oil.
Bake 45–60 minutes, until golden and bubbling.
Serve warm with vanilla ice cream.
