From Living on Live Food by Alissa Cohen, this recipe is one of my all-time favorites. I even made it for a Sample Saturday for the Co-Op I used to work at.
3 portobello mushroom caps, stem removed
.125 teaspoon sea salt
Blend avocado, tomato, onion and salt in food processor (or chop finely and mash together) until smooth. Turn mushrooms stem-side up and remove stems. Fill with blended filling.
I just came across your posting of this recipe. It sounds Great! Simplicity at it's finest! Thanks for sharing. Cheers!