For the filling:
Folding whipped cream into the dry ingredients gives these biscuits a tender, fluffy texture. Adding rhubarb to the filling gives it a seasonal twist.
Mix berries with ¼ cup sugar and 2 tablespoons orange juice in a medium-size bowl. Toss gently and set aside. In a small saucepan, combine rhubarb, remaining ½ cup sugar, and salt. Set over medium heat and cook, stirring, until sugar melts and liquids are simmering nicely, 8 to 10 minutes. Remove from heat and cool to room temperature. Rhubarb will soften as it sits. Add berries and stir. Set aside. Preheat oven to 400°. Grease a baking sheet, and set aside. Meanwhile, make the biscuits:
For the biscuits:
In a large bowl, whisk together flour, baking powder, salt, sugar, and orange zest. In the bowl of a standing mixer, whip cream until it forms firm peaks. Fold whipped cream gently into flour mixture until dough begins to come together. (Add another tablespoon or two of unwhipped cream if needed.) Gently gather dough into a ball and turn out onto a floured surface. Press to form a disk about ¾ inch thick. It will be slightly crumbly.
Using a 2-½-inch-wide biscuit cutter, cut biscuits and lay them on baking sheet. Brush with extra cream and sprinkle with coarse or granulated sugar. Bake until golden, 15 to 17 minutes. Cool on racks.
For the whipped cream:
Just before serving, make the whipped cream: Beat cream, sugar, and vanilla until firm peaks form.
To plate, split each biscuit horizontally. Spoon berry and rhubarb mixture on bottom half; cover with top half. Garnish with whipped cream.