Preheat your oven to 400°F (200°C) and line a large rimmed baking sheet with parchment paper for easy cleanup.
Cut the tempeh into 1-inch cubes and break the cauliflower into small, bite-sized florets.
In a large mixing bowl, combine the tempeh and cauliflower with olive oil, garlic powder, smoked paprika, sea salt, and black pepper, tossing until every piece is well-coated.
Spread the mixture in a single layer on the prepared baking sheet and roast for 20-25 minutes, flipping halfway through, until the cauliflower is tender and the tempeh is golden and crisp.
While the tray roasts, whisk together the tahini, lemon juice, and water in a small bowl until the sauce reaches a smooth, pourable consistency.
Transfer the roasted vegetables and tempeh to a serving plate, generously drizzle with the lemon-tahini sauce, and finish by sprinkling the hemp hearts over the top.