Broil 5 medium tomatoes (about 1 3/4 lb.); 3 serrano chile peppers; 2 garlic cloves, peeled; and 1 medium onion, sliced, on an aluminum foil-lined baking sheet 5 inches from heat 6 minutes on each side or until peppers are blistered. Remove tomatoes, chile peppers, and garlic from baking sheet; broil onions 4 more minutes on each side. Remove from oven; let stand 15 minutes. Remove peels and stem ends from tomatoes and chile peppers. Remove seeds from chile peppers. Pulse tomatoes, chile peppers, garlic, and onion in a blender 10 times or until chunky. Stir in 1 tsp. salt and 1/4 cup chopped fresh cilantro.