Break the cauliflower into similar sized, fairly large pieces, including stems. Steam until really soft, about 10- 15 minutes. (you need the cauliflower to be softer than you'd normally eat it, when in pieces.)
Meanwhile, in a heavy fry pan, toast the cumin seeds, shaking the pan often to ensure they don't burn. Remove from heat and roughly grind with a mortar and pestle. (optional). Add 1 T olive oil to pan and saute the onions till translucent. Add the cumin seeds back to the pan, along with the garlic, and cook a few minutes more. Place the cauliflower in a soup pot, and cover with the stock. Throw the onion and garlic mix into the pot and bring it all to a gentle simmer. At this point you can use a potato masher to mush up the cauliflower, or you can blend some or all of the soup in a blender. Add the tahini when the soup is at the desired consistency, and be careful not to let the soup boil after adding this. Add black pepper to taste and salt if necessary.

Serve with extra olive oil and toasted sourdough.