Preheat oven to 400°. Line baking sheet with foil.
Cut the potatoes in half lengthwise. Cut each half into wedges, about 1 inch thick, 4 pieces per half.
Place the potatoes and 1 tsp of salt in a medium-sized pot. Cover with cold water.
Quarter 1 lemon. Squeeze the juice into the pot, then add the lemon rinds to the water. Add the garlic. Bring to a boil & cook for 5 minutes. Drain and discard the lemon & garlic.
Place the potatoes on the baking sheet. Add the grapes, drizzle with the oil, and sprinkle with the rosemary and the remaining ½ tsp salt. Season to taste with black pepper. Toss to coat the potato wedges and grapes.
Squeeze the juice of the remaining ½ lemon over the potato/grape mixture. Roast until most of the grapes have burst open slightly. About 45 minutes. Serve.