Preheat the oven to 400 F (200 C).Line a 12-muffin tin with paper cups. Set aside.In a bowl, combine the flour, baking powder, salt, and brown sugar.Add the sliced apples and raspberries and mix gently until the fruits are just coated.In another bowl, mix the buttermilk, eggs, and melted butter until the mixture is smooth.Pour the mixture into the previous mixture and mix gently until no dry ingredients are visible.Divide the dough into the prepared cups.The dough will fill the cups well but will not overflow during baking.Sprinkle the tops of the muffins with turbinado sugar, then bake on the middle rack for 20 minutes or until a toothpick inserted into the center of the muffins comes out clean.Remove from the oven and let rest for 5 minutes before transferring to a cooling rack.