Method:
Cut tempeh in half and steam in or over simmering water for 20 minutes. Remove from heat and transfer to a small plate. Poke small holes in tempeh with a fork on both sides and set aside.
Mix tamari, mirin, garlic powder, and onion powder in a wide, shallow dish to make a marinade. Add tempeh, turning to coat, cover, and set aside for at least 30 minutes or up to overnight.
Heat a grill over medium heat. Grill tempeh about 4 to 5 minutes on each side, or until browned with grill marks. Alternately, heat a small amount of canola oil in a skillet over medium heat and cook tempeh for 3 to 4 minutes on both sides, until browned.
Serve tempeh on buns with your choice of toppings.