Put the first four ingredients in the food processor and finely chop, then add oil and process until paste-like. Then add the pistachios and process until the mixture is as coarse or as fine as you like. Add avocados and process until fairly smooth--they won't take much. Finally, as above, taste for salt and add, process, and taste again.
You'll almost certainly want to make this ahead of time, and since it has avocado in it you need to protect it from air or it will turn dark. So put it into a bowl and brush a little lime juice over the top. Then press plastic wrap directly on it. You can then put a regular cover on the bowl, but having a barrier directly on the surface is the only way to guard against darkening because that's the only way to keep the air off of it. BTW, I'll allow myself a little indulgence here in which I address the "bury the avocado pit in the guacamole to keep it from darkening" old wives' tale. That only works for the little portion of the dip that's covered by the pit. So you can do it if you want to, as it won't hurt anything, but you can't depend on it to take of the whole kahuna.
*Pati Jinich of "Pati's Mexican Table" on Create TV says that cilantro stems have a lot of flavor. So when I made this recipe a second time I just cut off and discarded the lower cilantro stems at the point where the leaves started and threw the top part into the food processor. (Give the leaves a good rinse first--they can harbor sand. I use my salad spinner to do this.) Much, much easier than picking off the leaves! Also, if you don't like cilantro, or would like a slightly different flavor, you can use part parsley. But really, cilantro is the way to go!
**Yes, these are expensive. Best way to buy them is in the bulk foods aisle of a natural grocery store such as Sprouts, or, if you can find them shelled, at a warehouse club such as Costco. The original recipe called for raw, unsalted pistachios, but for the most recent party I spotted a bag of shelled, roasted and salted ones at Costco and buying them was going to save me an extra stop at another store, so I went ahead and bought them. I thought that the dip had a greater depth of flavor made with this ingredient, but you could use whichever version you can find. If you do use the salted ones, be sure to leave out the salt until you have a chance to taste the mixture. Remember, even if you have to get an expensive ingredient in order to make something yourself you're still going to be paying less than you would if you bought the item pre-made--not that you'd be able to find this particular item in any store.