Make the filling: In a large bowl, whisk together the sugar, flour, salt, lemon juice and zest, and the vanilla bean seeds.
Add the blueberries and peaches and use a spatula to toss until well coated.
Transfer the filling to a 10-inch (25 cm) cast-iron skillet. Refrigerate until ready to top.
Make the biscuits: In a large bowl, whisk together the flour, granulated sugar, baking powder, and salt.
Using a cheese grater, grate the cold butter into the flour mixture. Working quickly, use your hands to toss the butter in the flour.
Pour 1 cup (245 ml) of buttermilk into the flour mixture and use a spatula to stir into a shaggy dough.
Turn the dough out onto a lightly floured surface. If the dough looks too wet, dust with a bit more flour. Form the dough into a 5x7-inch (12 x 17 cm) rectangle, and sprinkle with more flour. Fold the dough in half, then roll out again. Repeat 4–5 more times to build the flaky layers of the biscuits.
Add a few tablespoons of flour to a small bowl. Dip a 2-inch (5 cm) round cookie cutter in the flour, then punch out 16 biscuits, pressing the cutter straight down. Reroll the dough as needed to cut out more biscuits.
Arrange the biscuits on top of the fruit filling, making sure they are touching. Brush the biscuits with the remaining tablespoon of buttermilk and sprinkle with the turbinado sugar. Refrigerate until ready to grill.
Preheat the grill to 350°F (180°C).
Place the cast-iron skillet on the grill and shut the lid. Grill the cobbler for 1 hour 15 minutes, until the biscuits are golden brown and the filling is bubbling.
Let sit for 30 minutes at room temperature, then serve with ice cream or whipped cream.