Preheat oven to 375°F (190°C). In a large bowl, combine strawberries and rhubarb with 3/4 cup sugar. Toss cornstarch with a bit of sugar, then add to fruit with lemon juice, vanilla, and salt. Let sit 10 minutes.
Whisk flour, 2 tablespoons sugar, baking powder, and salt. Cut in cold butter until mixture resembles coarse crumbs. Stir in milk until just combined; dough will be sticky.
Pour fruit and any juices into a 9x13-inch baking dish. Drop spoonfuls of dough over fruit, leaving gaps. Beat egg and brush over biscuits if desired.
Bake 35–45 minutes until topping is golden and filling bubbles. Tent with foil if topping browns too quickly. Cool 10 minutes before serving.
Notes
Adjust sugar in the filling to match your fruit’s sweetness.
Use cold butter and do not overmix the topping to keep it tender.
Frozen fruit can be used straight from the freezer; add a few extra minutes to baking time.
Serve warm with vanilla ice cream or whipped cream.