Preheat oven to 425°F and line a baking sheet with parchment paper. Spray with olive oil.
Wash cauliflower, trim the bottom, but keep the core. Slice into ¾-inch steaks, roasting any florets that fall apart.
Lay the steaks and florets on the baking sheet. Spray tops with olive oil and sprinkle with half the seasonings.
Bake for 15 minutes, then remove and carefully turn the steaks. Spray with oil and add remaining spices.
Return to the oven and bake for another 10-15 minutes until browned and tender.
For the tahini sauce, mash garlic with salt, then whisk in tahini, lemon juice, and water until thick.