For the blueberry filling:
6 cups (960 grams) blueberries
1/3 cup (67 grams) granulated sugar
1 tablespoon cornstarch
1 teaspoon finely grated fresh lemon zest
Juice from 1/2 medium lemon (about 4 teaspoons)
1/2 teaspoon finely grated fresh orange zest
1/4 teaspoon freshly grated nutmeg
For the whole-wheat biscuits:
1 cup (130 grams) whole-wheat flour
1/2 cup (63 grams) all-purpose flour
1/4 cup (50 grams) granulated sugar
2 teaspoons baking powder
1/4 teaspoon baking soda
1/4 teaspoon fine salt
1/2 cup (120 milliliters) heavy cream
1/4 cup (120 mililitersl) low-fat buttermilk
1/2 teaspoon vanilla extract
1/4 cup (4 tablespoons) unsalted butter, melted and cooled, plus more for coating the skillet
2 teaspoons turbinado sugar
Vanilla ice cream or whipped cream, for serving (optional)
Arrange a rack in the center of the oven and heat to 375°F (190°C/Gas Mark 5). Coat a 10-inch cast iron skillet with butter.
Make the blueberry filling: Place the blueberries, sugar, cornstarch, lemon juice and zest, orange zest, and nutmeg in a large bowl and stir gently to combine. Pour the blueberry mixture into the skillet.
Make the biscuits: Whisk the flours, sugar, baking powder, baking soda, and salt together in a large, clean bowl; set aside.
In a measuring cup, combine the buttermilk, cream, and vanilla. Add the 1/4 cup melted butter and stir gently to combine.
Make a well in the center of the dry ingredients and pour in the buttermilk mixture. Use a rubber spatula to stir the ingredients together until just combined. The biscuit dough will be soft and sticky.
Use 2 spoons or a spring-loaded ice cream scoop to dollop the biscuit dough on top of the blueberries in 8 even pieces. The dough will rise and expand in the oven, so leave a bit of room in between the pieces. Brush the tops of the biscuits with the dregs of liquid left in the measuring cup, then sprinkle with the turbinado sugar.
Place the pan on a large baking sheet to catch any drips and slide the pan into the oven. Bake until the biscuits are golden and cooked through and the fruit is bubbling, 30 to 35 minutes. Serve warm with vanilla ice cream or whipped cream.