Preheat the oven to 400Â°F. In a small pot, combine the soy sauce, brown sugar, sake, sesame oil, garlic, and ginger over low heat. Swirl the pot around occasionally and cook for about 8 minutes, or until thickened. Lay the mushroom slices in a single layer on a cookie sheet lined with aluminum foil. Brush the portobello mushroom slices with the soy glaze. Bake for 8 to 10 minutes, until the mushrooms brown and shrink slightly.
Dab your fingers in a little water and grab about 2 tablespoons of sushi rice. Gently squeeze it into your palm to make it a compact mound. Put a portobello slice on top of that and wrap it with a band of nori, moistening the ends to seal. Garnish with the sesame seeds.
Edited: October 9th, 2009