Cut tempeh into 4 triangles.
Steam tempeh using a rice cooker or steamer basket on the stovetop. To steam using your rice cooker, add 1 cup of water to the rice cooker pot. Then, place the tempeh triangles in the steamer basket. Close the lid and press the steam or steam cook button (depending on your brand of rice cooker) and select 10 minutes for the steaming time. To steam using a steamer basket, boil 1-2 inches of water in a pot that fits your steamer basket. Place the tempeh triangles in the basket and carefully place the basket into the pot. Cover and steam for 10 minutes.
For either method, after steaming, turn off the heat and allow the tempeh to cool for about 5 minutes before removing from the rice cooker or steamer basket.
While tempeh is steaming, make the marinade. Use a shallow container that has a tight fitting lid and that is just big enough to fit the tempeh. I used a 7.5 x 5 x 1.5 inch rectangular glass Pyrex dish with lid that holds 3 cups (750 ml.) of liquid. Add tamari, olive oil, rice vinegar, garlic powder and onion powder, and then whisk together with a fork.
Carefully place each triangle into the marinade in the container. Then, carefully flip each piece so that the other side gets covered as well. You can use your hands or gently use a fork to do so.
Then, seal the container with the lid. Hold the container with two hands with your thumbs tightly securing the lid and gently shake up and down and side to side a few times to fully coat the tempeh.
Refrigerate for at least 30 minutes. You can flip the tempeh triangles again halfway during marinating, if desired, but it’s not entirely necessary.