Heat the oven to 375°F and butter a 9-by-13-inch baking dish.
Fold the strawberries and rhubarb with 1 cup sugar, cornstarch, vanilla and 1/4 teaspoon salt in the dish.
Whisk the flour, 1/3 cup sugar, baking powder and 1/2 teaspoon salt, then work in the butter until crumbly.
Stir in the cream only until the flour is moistened.
Drop the dough by heaping tablespoons over the fruit, keeping open spaces between the biscuits.
Bake for 43 to 47 minutes until the center bubbles and the topping is golden; rest 30 minutes so the filling can thicken.