Step 1: Preheat your oven to 400°F (200°C). Line a large baking sheet with parchment paper for easy cleanup.
Step 2: In a large bowl, combine the sliced tempeh with broccoli florets, bell pepper strips, and red onion wedges. Drizzle with 2 tablespoons of olive oil, soy sauce, and a pinch of salt and black pepper. Toss gently to coat all ingredients evenly. Spread the seasoned tempeh and vegetables in a single layer on the prepared baking sheet. Ensure they are not overcrowded; use two baking sheets if necessary to allow for proper roasting and browning.
Step 3: In a small bowl, whisk together the tahini, warm water (start with 2 tablespoons and add more gradually), lemon juice, maple syrup, ground cinnamon, garlic powder, and a pinch of salt until smooth and pourable.
Step 4: Bake the tempeh and vegetables for 20 minutes. Remove the baking sheet from the oven, flip the tempeh and vegetables, and return to the oven for another 15-20 minutes, or until the tempeh is golden brown and the vegetables are tender-crisp with slight charring.
Step 5: Once roasted, transfer the tempeh and vegetables to serving plates. Drizzle generously with the cinnamon tahini sauce. Garnish with fresh cilantro and sesame seeds, if desired. Serve immediately with fresh lemon wedges.