Steam the tempeh (optional): Place the block of tempeh in a steamer basket and steam for 10 minutes. This step helps remove the bitter taste characteristic of tempeh. Slice the steamed tempeh into 8 pieces.
Prepare the tempeh marinade: Whisk together all of the marinade ingredients in a shallow dish or pie plate until smooth. Add the tempeh pieces and coat thoroughly in the marinade. Refrigerate for at least 2 hours or up to overnight.
Bake the tempeh: Preheat the oven to 375°F (or 190°C). Place the tempeh in an ovenproof dish or on top of a baking sheet and bake for 12-15 minutes, then flip the pieces over and bake for another 5-10 minutes.
Prepare the veggie slaw: Combine the sliced carrots, bell pepper, Napa cabbage, scallions, and cilantro in a bowl. Top with the sunflower seeds and sesame seeds.
Make the dressing: In a small bowl, whisk together all of the ingredients until smooth. Pour the dressing over the vegetables and toss well to coat all of the vegetables.
Serve the veggie slaw over the cooked brown rice and with the baked tempeh pieces on top.