Heat the oven to 375°F and butter a 9-by-13-inch baking dish.
Toss the frozen berries with 3/4 cup sugar, cornstarch, lemon juice, zest and 1/4 teaspoon salt in the dish.
Whisk the flour, cornmeal, 1/3 cup sugar, baking powder and 1/2 teaspoon salt, then cut in the butter.
Fold in the buttermilk until the dough is evenly moistened but still rough.
Spoon the dough in small mounds over the frozen fruit, keeping the center partly exposed.
Bake for 48 to 52 minutes until the center filling bubbles for a full minute and the topping is golden; rest 30 minutes.