Preheat a grill to high. Halve and seed 6 plum tomatoes and 1 red bell pepper; halve 1 small onion. Grill, turning once, until softened, about 8 minutes. Transfer to a food processor and puree. Transfer to a saucepan, add 1/3 cup brown sugar, 1/4 cup apple cider vinegar and 1 teaspoon salt and bring to a boil over medium-high heat; lower the heat and simmer, stirring, until thickened, about 45 minutes. Let cool.