- Remove brown gills from the undersides of mushrooms using a spoon; discard gills.
- Remove stems, if necessary, and reserve for another use, if desired.
- Melt butter in a large skillet over medium heat; add leek and garlic, and saute until tender.
- Remove from heat, and cool.
- Stir together avocado and next 3 ingredients in a medium bowl; stir in leek mixture.
- Press goat cheese evenly into mushroom caps; top evenly with avocado mixture.
- Sprinkle with walnuts, and drizzle with olive oil.
- Place on rack in a broiler pan.
- Bake at 400 °F for 5 minutes; cover loosely with aluminum foil, and bake 5 more minutes.
- Serve immediately.
- Garnish, if desired.