Set oven to 400/2o4C.
Place serrano pepper and garlic cloves (with skin) on a baking sheet lined with parchment paper or foil. Bake for 15 minutes until the outside is golden brown.
Chop off the top of the serrano pepper (and adjust heat by removing some of the white seeds), and peel the garlic. Place all the ingredients in a food processor and blend until smooth.