Preheat oven to 350 degree F. Saute onion with oil and sugar over medium-low heat until onion is soft and transparent (4 to 5 minutes). Add tofu and salt; lightly brown tofu. Remove from heat.
Place mushroom, stem side up, on a baking sheet. Mound filling over stem side of mushroom, dust with nutmeg. Bake 10 to 15 minutes on middle oven rack. Garnish with mint, parsley or cilantro. To serve, slice into wedges.