Steam the Tempeh for 15 minutes, and then let it cool. Cut the Tempeh in half lengthwise, and then cut it into 1 1/2-inch pieces. Pour teriyaki sauce into a shallow container and add the Tempeh slices. Marinate in the refrigerator, stirring occasionally, for at least 1 hour. Meanwhile, trim any rough ends from the stems of the mushrooms, Steam the sweet potatoes until just done, let cool, then cut into 1-inch chunks. Parboil the onions, then cut into quarters. Alternately thread the Tempeh, mushrooms, sweet potatoes, onion, pineapple and peppers on skewers. Brush with teriyaki sauce and grill for about 10 minutes, turning several times to cook evenly. Serve over brown rice.