Place roma (plum) tomatoes and serrano peppers in a medium saucepan over medium high heat with enough water to cover. Bring to a boil. Boil 15 minutes. Remove from heat and drain.
Place roma tomatoes, serrano peppers, onion, cilantro and garlic in a blender or food processor. Puree about 30 seconds, or to desired consistency. Transfer to a medium serving bowl, and salt to taste.
