Place a rack in the center of your oven and preheat the oven to 425 degrees F. Line a rimmed baking sheet with parchment paper.
Prepare the strawberries for topping: In a small bowl, stir together the sliced strawberries, granulated sugar, and Grand marnier. Refrigerate until you are ready to serve.
Make the shortcakes: Cut the butter into a small dice and place it in the freezer. In a small bowl or liquid measuring cup, whisk together 1/3 cup milk and Greek yogurt and place it in the refrigerator.
In the bowl of a food processor fitted with a steel blade, pulse together the 1:1 flour blend, 1 cup of the oats, brown sugar, baking powder, and salt. Briefly pulse twice to combine. Scatter the butter pieces over the top. Pulse until mixture forms crumbs the size of large peas. Pour in the milk/Greek yogurt mixture and pulse until the dough is just combined and holding together. It will be crumbly but should hold together when pressed. If it seems too dry, sprinkle another tablespoon of milk over the top and pulse again. Do not over mix and stop pulsing as soon as the dough comes together.
On a lightly floured surface, pat dough into 3/4 inch thick rectangle. Brush lightly with milk. Gently lift one of the shorter sides of the rectangle and fold the dough in half, as if you were closing a book (you want the dough to form a rough square). With a serrated knife, cut dough into 6 squares. Transfer the cut shortcakes to the prepared pan. Brush lightly with more milk, then sprinkle with the remaining 1 tablespoon oats and sparkling sugar.
Bake the shortcakes for 18 to 20 minutes, until nicely golden. Transfer to a wire rack and let cool for 10 minutes.
To serve: split each shortcake in half. Stir the strawberry mixture then spoon a generous portion over the top. Add a dollop of three of whipped cream. DEVOUR.
Store leftovers at room temperature for up to 2 days or freeze airtight for up to 2 months. Let thaw overnight in the refrigerator.
The macerated strawberries taste even better as they sit and can be prepared 1 day in advance. Leftover berries will keep for a few days in the refrigerator.