Preheat broiler to high; set a rack 4 inches from heat source. Arrange tomato halves cut side down on a broiling pan. Broil, watching carefully and turning pan as needed, until tomatoes are blackened and charred, about 10minutes. Transfer to a plate and let cool.
Place bell pepper and jalapeños on broiling pan; broil until blackened and charred, 7 to 10 minutes. Remove to a bowl, cover tightly with plastic wrap and set aside for 10 minutes. Peel pepper and jalapeños, but leave blackened skins on tomatoes. Coarsely chop tomatoes, pepper and jalapeños.
Working in batches if necessary, combine cooked tomatoes, bell pepper and jalapeños with onion, garlic, 1/2 tsp. salt and 1/4 tsp. pepper in a food processor. Pulse briefly until coarsely chopped. (Do not overprocess. The salsa should be chunky.) Pulse in cumin and cilantro. Season with additional salt and pepper. Cover and refrigerate until ready to serve.