BBQ Shrimp Cocktail

Name: BBQ Shrimp Cocktail
Ingredients: 8 plum tomatoes, quartered (about 1 1/2 pounds); 1/2 small Vidalia onion, cut in half through the root; 1 teaspoon vegetable oil; 1 tablespoon dark brown sugar; Kosher salt; 1 teaspoon ancho chile powder; 1 teaspoon freshly ground black pepper; 1/4 teaspoon ground allspice; 1/4 teaspoon ground cinnamon; 1/4 teaspoon dried mustard powder; Pinch cayenne; 1 bay leaf; 2 tablespoons cider vinegar; 2 tablespoons lemon juice; 1 tablespoon horseradish, prepared or freshly grated; 1 pound large shrimp, peeled and deveined, tail on; Special equipment: 4 long metal skewers