Preheat oven to 400 degrees Fahrenheit (200 degrees Celsius).
Remove outer leaves from cauliflower and slice into 1-inch thick steaks from the center.
Place cauliflower steaks and chickpeas on a large sheet pan.
Whisk 3 tablespoons olive oil with cumin, paprika, salt, and pepper; brush onto cauliflower and toss with chickpeas.
Roast for 25 to 30 minutes, flipping cauliflower halfway through, until tender and edges are browned.
Prepare green tahini by blending tahini, parsley, cilantro, garlic, lemon juice, remaining oil, and water until smooth.
Serve cauliflower steaks topped with roasted chickpeas and a generous drizzle of green tahini sauce.