Preheat the oven to 400˚ and get a pot of salted water (large enough to fit the head of cauliflower) boiling.
Place the head of cauliflower upside down into the boiling water until slightly softened, about 2 minutes.
Strain in a colander, then put the cauliflower upright on a rimmed baking sheet, evenly coat in olive oil (be generous), and bake in preheated oven for 1 hour. Cauliflower will be done when it's brown and you can slide a knife through it.
While the cauliflower is baking, combine all the ingredients of the tahini mixture in a large bowl.
Let the cauliflower cool to room temperature before breaking into bite-sized chunks and tossing in the tahini mixture, adding more olive oil or lemon juice if it looks dry.
Sprinkle with green onions or cilantro and coarse salt and serve.