1. Preheat oven to 400°F (200°C) and spray an 11 x 7-inch (2-quart) baking dish with nonstick spray.
2. Toss strawberries, blueberries, sugar, flour, balsamic, and salt together in a medium bowl. Set aside.

Biscuit Topping

1. Whisk together flour, sugar, baking powder, baking soda, and salt in a large bowl.
2. Dice butter into pieces; sprinkle over flour mixture and use a pastry cutter to cut in evenly until mixture resembles coarse meal.
3. Combine yogurt, milk, and vanilla in a small bowl; add to flour mixture and stir just until moist.
4. Lightly flour hands and knead mixture 3 or 4 times in the bowl, just until dough comes together.
5. Pour berry mixture into prepared baking dish; top with spoonfuls of biscuit dough.
6. Bake for 18 to 20 minutes, until juices are bubbling and biscuit topping is golden.
7. Let cool slightly on a wire rack before serving. Store leftovers covered in the refrigerator; warm before serving.