In a large bowl, whisk together brown sugar, 1/2 cup white sugar, corn starch and nutmeg. Add peaches and nectar; stir until homogenous. Pour into a 9x13" glass baking dish sprayed with baking spray. In separate large mixing bowl, whisk together flour, buttermilk powder, remaining sugar, baking powder and salt; using a pastry cutter, cut in the butter until the dough resembles wet sand. Slowly add water, a little at a time, while using a fork to lightly mix the dough just until it comes together (you may not use all of the water). Drop 12 spoonfuls of dough evenly over the filling. Brush top of dough with beaten egg; sprinkle generously with turbinado sugar. Bake in a preheated 375° oven until the topping is golden-brown (about 40 min.). Let stand 15-30 min. before serving.