Set your stove to broil with the oven rack four to five inches from the top.
Line a rimmed cookie sheet with foil and spread a little olive oil or cooking spray on it. Place the tomatoes and jalapenos on the sheet, open side up, along with the whole serrano pepper and the garlic cloves. Broil for five to six minutes – until things start to get a little brown. Turn everything over and grill for another five to six minutes with the skin up. The tomato and pepper skins should start to brown and ‘blister’ a bit. Remove from oven and cool a few minutes. Remove most of the tomato skins. Squeezing the garlic clove should extrude the garlic from the peel easily – you may have to cut off the thick end.
Place 1/2 of the tomatoes, the peppers, the garlic, the cumin, and the salt in a food processor, along with pan juices and puree, until smooth. Add the remaining tomatoes, the onions, and cilantro to the puree and ‘pulse’ the food processor a couple of times to blend the ingredients while keeping some texture in the salsa. Serve with tortilla chips or on tacos, etc.