Heat the oven to 425°F and line a baking sheet. Toss the cauliflower and chickpeas with one tablespoon of olive oil, cumin, smoked paprika, and half the salt.
Spread the cauliflower and chickpeas in one layer and roast for 25 to 30 minutes, turning once, until the edges are browned and the chickpeas are crisp.
Meanwhile, combine the rinsed quinoa with one cup water in a saucepan. Bring it to a boil, cover, reduce the heat, and simmer for 15 minutes. Rest covered for 5 minutes, then fluff.
Whisk the tahini, lemon juice, garlic, remaining olive oil, remaining salt, and water until smooth. Add a splash more water if needed for a pourable dressing.
Cool the quinoa for a few minutes, then toss it with the roasted cauliflower, chickpeas, parsley, and red onion.
Drizzle with the tahini dressing, toss gently, taste, and adjust the salt before serving.