In a medium mixing bowl, combine the raspberries with 1/4 cup sugar and set aside to macerate at room temperature, about 1 hour.
While the raspberries are macerating, assemble the biscuit dough: In a food processor, combine the flour, cornmeal, baking powder, baking soda, remaining 1 tablespoon sugar and the sea salt. Pulse to combine and then add the butter. Pulse until the butter has flaked into small pieces. Add the buttermilk and pulse until just combined. Remove the dough from the processor and set aside.
Add the lemon zest and cornstarch to the raspberries, stir to combine, and place the mixture in a 6 x 8 inch baking dish. Dollop spoonfuls of the biscuit topping over the raspberries. Bake for 35 minutes, or until the topping is golden brown and the fruit is bubbly. Serve warm, or at room temperature.