Pulse onion and Italian frying pepper or Cubanelle in the food processor until coarsely chopped.
With the motor running, add remaining ingredients, one at a time, until mixture is smooth.
Sofrito keeps 3 days in the refrigerator; freeze for longer storage (can be used without thawing).
NOTES: Ajices Dulces, also known as cachuchua or ajicitos, are tiny peppers with only a hint of heat. They look similar to scotch bonnets or habaneros, so don't get them confused! If you cannot find them , use a little more cilantro and a pinch of cayenne or crushed red pepper flakes.
NOTE: Culantro has long tapered leaves with serrated edges. If you cannot find it, just leave it out.