In a large bowl, combine 1 pound uncooked red potatoes, scrubbed and thinly sliced (about 3 cups), 1 medium yellow pepper, cut into thin strips (about 2 cups), 1 medium red onion, thinly sliced (1 cup), 1 Tbsp olive oil, 2 minced medium garlic cloves, 1 Tbsp fresh chopped rosemary, 1/2 tsp salt, and 1/2 tsp freshly ground black pepper. Mix until coated. Tear off four 12-inch long pieces of aluminum foil. Spread about 1 1/2 cups of vegetable mixture in a fairly even layer near one end of each piece of foil. Fold foil over to form packets; fold edges over to form a tight seal. Grill packets, turning a few times, 20 to 25 minutes.