2 cups old-fashioned rolled oats
2 cups whole-wheat flour
2 sticks (1 cup) cold unsalted butter, cut into pieces
1/3 cup packed dark brown sugar
1 3/4 teaspoons baking powder
1 1/2 teaspoons salt
1/2 cup whole milk
Coarsely chop oats in a food processor and transfer to a large bowl. Pulse together flour, butter, brown sugar, baking powder, and salt in processor until mixture resembles coarse meal. Add milk and blend until mixture just forms a dough. Add to oats and knead until just incorporated. Halve dough and pat each half into a 5- by 3-inch rectangle. Chill, wrapped well in plastic wrap, until firm, about 3 hours. Preheat oven to 350F. Cut 1 rectangle crosswise into scant 1/4-inch-thick slices and bake on an ungreased large baking sheet in middle of oven until undersides are a shade darker, about 20 minutes. Transfer to a rack to cool and make more biscuits with remaining dough.