Pulse the oats in a food processor until finely ground. Add the flour, brown sugar and salt and pulse. Add the butter and egg yolk and process until the dough comes together and no streaks of butter remain. Divide the dough between 2 sheets of plastic wrap and pat into disks; wrap and refrigerate until firm, about 3 hours.
Position racks in the upper and lower thirds of the oven; preheat to 350 degrees F. Line 2 baking sheets with parchment paper. Let the dough sit at room temperature about 5 minutes to soften slightly. Working with one piece at a time, roll out the dough on a lightly floured sheet of parchment paper until 1/4 inch thick; dust with more flour. (If the dough becomes too soft, refrigerate 15 minutes.) Cut out cookies with a 2 1/2-inch round cutter (preferably fluted), then cut out the centers with a 3/4-inch round cutter. Arrange the wreaths 1 inch apart on the prepared pans. Reroll the scraps and cut out more cookies.
Bake, switching the pans halfway through, until the cookies are dry and firm on top and golden brown around the edges, 15 to 20 minutes. Let cool 5 minutes on the pans. Lightly brush the tops with the maple syrup, then immediately sprinkle with the crushed raspberries and sanding sugar. Transfer to a rack to cool completely.