Preheat your oven to 425°F (220°C) and line a baking sheet with parchment paper.
In a large bowl, whisk together the flour, white sugar, baking powder, baking soda, and salt.
Cut in the cold butter with a pastry blender or fork until the mixture resembles coarse crumbs.
Fold in the rhubarb and strawberries gently.
Add buttermilk and stir just until combined – do not overmix.
Using a large spoon or cookie scoop, drop portions of dough onto the prepared baking sheet.
Sprinkle each biscuit with turbinado sugar for a sweet crunch.
Bake for 15-18 minutes, or until golden brown.
Cool slightly before serving. Enjoy warm or at room temperature.