Peel and mash avocados in a medium serving bowl. Stir in onion powder, garlic, tomato, lime juice, cilantro, salt and pepper. (Go easy on salt if you intend to use salted pistachios).
Stir in finely chopped pistachios right before serving. They give the guacamole a great texture and nuttiness that we love, but you can leave them out.
Garnish with a wedge of lime, additional chopped cilantro and a sprinkle of pistachios.