For the tomato relish, fold a 36x18-inch piece of heavy foil in half to make an 18x18-inch square. In a medium bowl combine 2 cups of the tomatoes, the onion, garlic, oregano, mustard, water, salt, brown sugar and pepper. Place mixture in center of the foil. Bring up two opposite edges of the foil; seal with a double fold. Fold the remaining edges to enclose the mixture, leaving space for steam to build.
For a charcoal grill, place the foil packet on a grill rack directly over medium coals. Grill for 10 minutes; turn packet. Grill about 10 minutes more or until onion is tender. (For a gas grill, preheat grill. Reduce heat to medium. Place foil packet on grill rack directly over heat. Cover and grill as above.).
Meanwhile, place hot dogs on grill rack around foil packet; grill for 5 to 7 minutes or until heated through, turning occasionally.
Carefully open rellish packet and stir remaining 1 cut tomato halves and the grapes into hot relish mixture. Place hot dogs in toasted buns; serve tomato relish with the hot dogs.