1 2/3 cups all-purpose flour, plus more for rolling
1 1/3 cups old-fashioned rolled oats, plus more for topping
1/4 cup plus 2 tablespoons light-brown sugar
1/2 teaspoon ground cinnamon
1/2 teaspoon freshly grated nutmeg
2 teaspoons baking powder
3/4 teaspoon baking soda
1/2 teaspoon salt
1 1/2 sticks cold unsalted butter, cut into pieces
1 1/2 cups diced Granny Smith apple (2 apples cut into generous 1/4-inch cubes)
2/3 cup cold buttermilk, plus more for brushing
Raw turbinado sugar, for sprinkling
Preheat oven to 400 degrees. Whisk together flour, oats, brown sugar, cinnamon, nutmeg, baking powder, baking soda, and salt. Cut in butter with a pastry blender or rub in with your fingers. (The largest pieces should be the size of small peas.) With your fingertips, flatten butter pieces into small disks. Add apples and buttermilk, stirring until dough just comes together.
Turn out dough onto a lightly floured work surface. Pat dough into a 6-by-8-inch rectangle, and cut into twelve 2-by-2-inch squares with a floured knife. Place about 2 inches apart on parchment-lined baking sheets. Brush tops with buttermilk, and sprinkle with raw turbinado sugar and oats. Bake until golden brown, 20 to 22 minutes, rotating sheets halfway through. Let cool on sheets for 15 minutes. Serve warm or at room temperature.