Preheat the oven to 200 degrees . In a medium skillet, cook the grapes, 2 tsp. EVOO and the granulated sugar over medium heat, stirring, until the grapes begin to soften, about 5 minutes. Remove from the heat and let cool.
Pour the remaining 2 1/2 tsp. EVOO onto a large, shallow, parchment-lined baking sheet, spreading to coat. Put the dough on the baking sheet; press to flatten. Turn the dough over, coating with the EVOO. Stretch the dough to a 9-by-12-inch oval. Spoon the grapes on top, leaving a 1-inch border. Turn off the oven; let the dough rise in the oven for 15 minutes. Remove from the oven; place in a warm spot to rise for another 15 minutes. Increase the oven temperature to 400 degrees .
Bake the focaccia until it is golden around the edges and the grapes are tender, about 20 minutes. Let cool for 15 minutes, blotting any grape juice on the pan. Transfer to a cutting board. Dust very lightly with confectioners sugar before serving.