Warm a dry skillet over medium heat. Add the tomatoes, pepper, and garlic and roast on all sides, turning with tongs as needed, until blackened in multiple spots.
Remove stem from pepper and add to blender along with tomatoes, garlic, onion, and cilantro. Pulse until chopped to desired consistency.
Return the same skillet to medium heat and add olive oil. Once the oil is hot, pour in the salsa and simmer for 10-15 minutes, until thickened.
Season to taste with salt and lime juice. Serve warm or let cool.
Store in an airtight container in the refrigerator.
Notes
Makes about 3 cups
For more heat add more peppers; for less heat use a jalapeño in place of the serrano.