Place onion, peppers, and garlic in a food processor and pulse to chop fine. Cut stems off of cilantro just below leaves. Add tomatoes and cilantro to processor and pulse to chop slowly so that tomatoes do not become too small.
Remove from processor and squeeze fresh lime over mixture, stir well. Add salt and stir well. Serve with tortilla or pita chips.
For spicier flavor use whole Serrano pepper. For milder flavor use more tomatoes or less peppers.