Fill a pan about ¾ with water. Bring to a boil, and then carefully place the cauliflower (head down) in the water. The base will stick out a little bit.
Roast in the oven for about an hour, then remove from the oven and spread the remaining oil-salt mixture over the cauliflower. Roast again for 30 min to an hour, depending on the size of the cauliflower.
Pour the tahini into a small bowl, followed by lemon juice, salt, and water. Mix to combine. Add more water and whisk to combine until the sauce is smooth and runny.