Place onion and jalapeno in a bowl. Rinse preserved lemon to remove salt. Mince peel, discarding pulp. Add minced peel to onions and jalapeno. Add salt, and allow to marinate for five minutes.
Add the tomatoes, avocado, and cilantro to the bowl and stir to combine. Taste, adding salt if necessary. (For extra punch, splash in some preserved lemon brine.)
Marinate pico de gallo for 15 minutes at room temperature or for several hours in the refrigerator. Will keep up to 4 days, covered, in the refrigerator.