Preheat your oven to 400°F (200°C) and line a large, rimmed baking sheet with unbleached parchment paper for easy cleanup.
Slice the tempeh into 1/2-inch cubes and place them in a large mixing bowl along with the drained chickpeas, broccoli florets, and cauliflower florets.
Drizzle the olive oil over the mixture, then sprinkle with sea salt, black pepper, and garlic powder, tossing well to ensure every piece is evenly coated.
Spread the vegetables and tempeh in a single layer on the prepared baking sheet, ensuring they aren't overcrowded so they roast rather than steam.
Roast for 20-25 minutes, tossing halfway through, until the vegetables are tender-crisp and the tempeh is golden brown and slightly firm.
While roasting, prepare the dressing by whisking together the tahini, lemon juice, nutritional yeast, and water in a small bowl until completely smooth and pourable.
Divide the roasted mixture into bowls and drizzle generously with the creamy, savory tahini dressing before serving warm.