Heat the oven to 375° F and line a baking sheet with Silpat or parchment paper.
In a large bowl, whisk together the flour, oats, 3 tablespoons of the sugar, salt, and baking powder. Add the butter cubes, toss to coat them in the flour, and then, working quickly, squash each one into a flat sheet, tossing with the flour as you go so as to keep all butter pieces protected from the heat of your hands. You're looking for a mixture that resembles a very coarse cornmeal.
Use a spoon to stir in the cream and egg until combined. Set your mixture in the freezer for 5 to 10 minutes, to chill and firm.
Lightly dust a work surface with flour. Roll out your chilled dough to a 10- by 6-inch rectangle. Place about half the raspberries (a good handful) over half the rectangle, then sprinkle with 1 tablespoon sugar. Fold the rectangle in half, then roll/pat—be gentle—into an 8- by 6-inch rectangle.
Place another handful of raspberries on half the rectangle, followed by the last tablespoon of sugar. Fold the dough in half again, then pat into a 6-inch square (it's okay to approximate). Use a very sharp, floured knife to cut your block into 6 pieces (squares or triangles—whatever you prefer). Place the shortcakes on a baking sheet and freeze for 20 to 30 minutes, so that your shortcakes have a chance to chill and firm up again. Then brush with cream, sprinkle with the turbinado sugar, and bake for 17 to 20 minutes, until golden and firm. (If your biscuits aren't browning, up the temp to 400° F). Let cool for 5 minutes on the pan, then transfer to a wire rack to cool completely.
For the assembly:
In a small bowl, mix the raspberries with sugar and, if using, vinegar. Set aside for 5 to 10 minutes, stirring occasionally, until the raspberries are juicy. Taste and adjust sugar as needed.
In a chilled bowl, whip the cream to soft peaks (I prefer very soft).
To serve the shortcakes, slice them in half, like sandwich buns. A serrated knife will help! Spoon whipped cream over the bottom half, followed by the macerated raspberries. Top with the shortcake hat. Devour!
A (former) student of English, a lover of raisins, a user of comma splices. My spirit animal is an eggplant. I'm probably the person who picked all of the cookie dough out of the cookie dough ice cream. For that, I'm sorry.