Recently when making any kind of fresh salsa or pico de gallo, I have been adding green papaya. I have long been looking for a good additive to fresh Mexican style hot sauces. Often they are just too much and need something to mellow them out and tone them down a little. A lot of cooks use tomatillo for this purpose but I don't like tomatillo. I've tried a lot of different stuff, like celery, cucumber, jicama, etc., with varying success. Often it's good but never great. You can either shred it fine or pulverize it in a food processor.
Green papaya is great. It absorbs and combines all the flavors from the other ingredients, lightens it up, and creates a more subtle flavor. It's great right after it's made but even better after several hours to a full day of melding.
I can get green papaya here in the Asian markets all the time. It grows just fine in SoCal and Mexico. It's cheap. I got on to green papaya because it is the basis of many of my favorite, spicy, South East Asian salads that feature salty, fermented crustacean products, lime juice and fiery hot chiles, requiring a lot of buffering.