Position rack in middle of oven. Preheat oven to 375°F/190°C.

For the Filling: Toss the rhubarb, strawberries, sugar, cornstarch and salt together in a 2-quart (2 L) ovenproof decorative casserole; set aside.

For the Cobbler Biscuits: Whisk the flour, cornmeal, sugar, baking powder and salt together in a mixing bowl to aerate and combine.

Add butter and cut in with a hand-held pastry blender, two knives or your fingertips until the butter is cut into assorted very small uniform pieces. Add milk and egg and stir until evenly moistened.

Dollop the biscuit batter on top of the fruit filling, allowing the fruit to peak out here and there. Bake for about 45 to 55 minutes or until filling is bubbling and the biscuits are golden brown.