Step 1: Preheat oven to 400°F (200°C). Line a large baking sheet with parchment paper. If desired, steam tempeh for 10 minutes, then pat dry and cut into cubes or triangles.
Step 2: In a medium bowl, whisk together 2 tablespoons of olive oil, soy sauce/tamari, smoked paprika, garlic powder, onion powder, and pepper. Add tempeh cubes and toss to coat. Let marinate for at least 10 minutes.
Step 3: While tempeh marinates, chop broccoli, bell peppers, and red onion into uniform, bite-sized pieces.
Step 4: In a large bowl, combine chopped vegetables. Drizzle with the remaining 1 tablespoon of olive oil, salt, and pepper. Toss well to coat.
Step 5: Spread the marinated tempeh and seasoned vegetables in a single layer on the prepared baking sheet. Ensure not to overcrowd the pan; use two sheets if necessary for proper roasting.
Step 6: Roast for 25-30 minutes, flipping the tempeh and vegetables halfway through. Cook until vegetables are tender-crisp and slightly charred, and tempeh is golden brown.
Step 7: While roasting, prepare the cinnamon tahini sauce. In a small bowl, whisk together tahini, starting with 3 tablespoons of warm water, maple syrup, lemon juice, ground cinnamon, and a pinch of salt. Add more water, a teaspoon at a time, until the sauce is pourable but still creamy.
Step 8: Once roasted, remove the sheet pan from the oven. Drizzle generously with the cinnamon tahini sauce.
Step 9: Garnish with fresh parsley or cilantro and sesame seeds, if desired. Serve immediately and enjoy your delicious sheet pan tempeh and vegetables with cinnamon tahini sauce!