Preheat oven to 425°F. Cut cauliflower into medium pieces, toss on a large rimmed baking sheet with the sliced onion, thyme, 4 unpeeled garlic cloves, and olive oil. Season with salt and pepper. Roast, tossing occasionally, until just starting to char, 35-40 minutes.
Sprinkle the cauliflower with the juice of 1 lemon, toss to combine, and roast until cauliflower is tender, 10-15 minutes longer.
In a small bowl, whisk together the tahini, lemon, oil and garlic. Whisk until smooth. Add the water and continue mixing until pourable and season with salt.
Drizzle the tahini sauce over the cauliflower and serve with extra lemon as needed.