1. Shortcakes: Preheat oven to 400 degrees. Butter a baking sheet.

2. In a large bowl, whisk together flour, oats, brown sugar, baking powder, lemon zest, and salt. Using a pastry blender, two knives, or your fingers, cut in butter until pieces are no larger than peas. Stir in cream until mixture is crumbly.

3. Turn mixture out onto a floured surface and knead until it holds together. Pat into a 3/4-inch-thick rectangle. Generously brush top with cream; fold dough in half like you are closing a book. Brush with more cream; cut dough into 6 squares. Transfer to prepared sheet, spacing 2 inches apart.

4. Bake until shortcakes are golden brown, 17 to 23 minutes. Transfer sheet to a wire rack; let cool completely. Increase oven temperature to 450 degrees.

5. Topping: Meanwhile, place grapes on a rimmed baking sheet, sprinkle with sugar and vinegar, and dot with butter. Add cinnamon and thyme. Roast, stirring once or twice, until grapes are caramelized, 15 to 20 minutes. Transfer sheet to a wire rack; let cool slightly.

6. In a chilled bowl, whisk cream and creme fraiche to soft peaks. To serve, split shortcakes in half horizontally and spoon some grapes onto bottoms. Sprinkle grapes with flaky salt and pepper, if desired, then add dollops of whipped creme fraiche. Cover with shortcake tops. Serve immediately.