In the bowl of a slow cooker, toss together 10 ounces blueberries, the chopped 6 nectarines, juice of 1 lemon, 1/4 teaspoon cornstarch, 1/2 teaspoon vanilla extract and 1/3 cup granulated sugar.
In a large mixing bowl, stir together 2 cups all-purpose flour, 1 teaspoon salt, 1 tablespoon baking powder and 3/4 cup powdered sugar Use a pastry cutter to cut in 1/2 cup butter until it resembles a course meal. Pour in 1 cup half and half and stir to form a thick batter.
Scoop the batter in large spoonfuls on top of the blueberry nectarine mixture.