Prepare grill for high-heat cooking (450+°F). Place tomatoes, onion, Serrano peppers and limes (cut side down) on a well-oiled grill grate. Grill tomatoes for approximately 7 minutes until charred on all sides and the skin is beginning to split. Grill onions and peppers for 4-5 minutes until charred on all sides and limes for 3–4 minutes until the cut sides are caramelized.
Roughly chop tomatoes and dice the onion and Serrano peppers (for a more mild salsa, remove the seeds and ribs from the peppers). Place tomato, onion, Serrano pepper, juice from the lime halves, and cilantro in to a blender or food processor and pulse until the ingredients are finely chopped. Add salt and pepper to taste then serve with tortilla chips.