Cook the brown rice for between 40 and 50 minutes or until tender. Drain and pour boiling water through the cooked rice to separate the grain.
Blanch the silverbeet in a large pot of boiling water for 1 minute before draining thoroughly and chopping into small pieces. Mix the silverbeet through the rice with the garlic and sesame seeds, and oil and season with sea salt and pepper.
Fill each mushroom with the rice mixture and place on a baking tray. Bake in the oven for 25 minutes.
Note: Sesame seeds were through to have originated in India as far back as 1600BC, where they represented a symbol of immortality.