Preheat your oven (or smoker or grill set up for indirect heat) to 350 degrees F. Grease a 10 to 12-inch cast iron skillet and set aside. If using the oven, place the skillet on a rimmed baking sheet.
Rub the orange zest into the sugar until fragrant. In a large bowl, combine the strawberries, peaches, orange sugar, tapioca, vanilla, orange juice, and salt. Stir to combine and let sit for at least 10 minutes while you make the biscuit topping.
Transfer the fruit mixture to the prepared skillet.In a medium bowl, whisk together the flour, sugar, baking powder, and salt. Using your fingers or a pastry cutter, work in the cold butter until the mixture resembles coarse crumbs with pea-sized pieces of butter throughout. Add the buttermilk and stir just until a shaggy dough forms. Do not overmix.
Drop the dough in generous ½-cup scoops over the fruit filling. Brush the tops with buttermilk and sprinkle generously with turbinado sugar.
Bake for 45 to 55 minutes in the oven (or 50 minutes on the grill or smoker, rotating the skillet halfway through) until the biscuits are deeply golden brown and the filling is bubbling at the edges.
Rest for at least 15 to 20 minutes before serving. Top each bowl with a generous scoop of vanilla ice cream and serve immediately.