Preheat your oven to 400°F (200°C) and line a large rimmed baking sheet with parchment paper.
Cut the seitan into bite-sized cubes and place them on the baking sheet along with the broccoli florets, sliced bell pepper, and diced red onion.
Drizzle the extra virgin olive oil over the seitan and vegetables, then sprinkle with the garlic powder, smoked paprika, sea salt, and black pepper.
Toss the ingredients thoroughly with your hands or a spatula to ensure an even coating of oil and spices, then spread them out into a single layer.
Roast in the oven for 18 to 20 minutes, tossing halfway through, until the seitan is golden-brown and the vegetables are tender with slightly charred edges.
While the bowl components roast, whisk together the tahini, lemon juice, and water in a small ramekin until the sauce is smooth and pourable.
Place the warm cooked quinoa in the bottom of a serving bowl, top with the roasted seitan and vegetable medley, and finish by drizzling the creamy tahini sauce over the top.