Separate the 21 ounces concord grapes from their stems. Rinse the grapes and pat them dry.
Preheat the oven to 375°F (190°C).
Generously slick a 8-inch-by-12-inch (20-cm-by-30-cm) baking sheet or round pizza pan with olive oil. Using well-oiled hands, divide the dough into 2 halves, one slightly larger than the other. Place the larger half onto the oiled pan and, using your fingers, spread the dough out evenly to cover the pan or so that it's no more than 1/2 inch (1.5 cm) thick.
Place about 2/3 of the grapes on the dough and sprinkle with half the superfine sugar followed by about 2 tablespoons (1 fluid ounce/30 ml) olive oil.
Stretch the remaining the dough to roughly the size of the pan and place it on the grapes, continuing to stretch it as needed to cover the grapes and dough beneath.
Roll up the edges of the bottom layer of dough from the underneath to seal the edges. Gently press down all over the surface of the dough to create little dimples. Arrange the rest of the grapes on the dough and evenly sprinkle with the remaining superfine sugar and 3 tablespoons (1 1/2 ounces/45 ml) olive oil.
Bake until the dough is golden and crunchy and the grapes are oozing, 30 to 35 minutes.
Let the grape focaccia cool completely. Cut it into squares. (If you like, you can dust it with confectioners sugar or coarse sea salt just before serving—although this isn’t exactly traditional, it is rather nice.) This is best served and eaten the day of baking.