Put the shallot, jalapeno pepper, and garlic in the bowl of a food processor fitted with the metal blade and process until finely chopped (a mini food processor is best but large will work too).
Transfer the mixture to a medium bowl. Add the chopped tomatoes, avocado, salt, lime juice, and cilantro to the bow and stir to combine. (If not using right away, see the make-ahead instructions below).