Bring the pork tenderloin almost to room temperature in a non-reactive container or gallon-sized zip-top bag. 

Place all ingredients for marinade in a blender or food processor and process until finely chopped. Reserve 1/2 cup of marinade. Pour remaining marinade over pork tenderloin and turn to coat. Allow to rest at room temperature for at least 20, and up to 30 minutes.

Meanwhile, heat a grill to medium.

Remove pork from marinade (discard marinade). Place it on the grill and cook, turning every two minutes, until internal temperature reaches 140˚F, 12 - 14 minutes. The marinade that clings to the pork will brown; don't allow it to burn. When pork is done, remove to a sheet of foil large enough to make a "boat" that will enclose the pork; add reserved marinade, seal the foil tightly and allow to sit for ten minutes. Slice thinly across the grain and serve at once.

Note: Serrano chilis tend to be hotter than jalapeno peppers. Use at your discretion.