Fold 36x18-inch heavy foil in half to 18 inches square. In a medium bowl, combine 2 cups of the tomatoes, the onion, garlic, mustard, marjoram, water, salt, brown sugar, and pepper. Place mixture in center of foil. Bring up two opposite edges of foil; seal with double fold. Fold remaining edges to enclose mixture, leaving space for steam to build.
For a charcoal grill:
Place foil packet on grill rack directly over medium coals. Grill for 10 minutes; turn packet. Grill about 10 minutes more or until onion is tender. Carefully open packet.
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. Remove hot dogs from grill.
Stir remaining 1 cup tomato halves and the grapes into hot chutney mixture. Place hot dogs in toasted buns; serve Texas Tomato Chutney with the hot dogs. Makes 6 servings.