Put the tahini, the 1/3 cup water, the tamari, lemon juice, cumin, and cayenne in a small bowl, and stir together. Put the vegetable stock and carrots in a large pot, cover, and bring to a boil over medium heat. Immediately reduce the heat, and simmer for 5 minutes. Stir in the broccoli, leeks, tomatoes, and parsley. Cover and return to a boil over medium-high heat, then immediately reduce the heat and simmer until the vegetables are tender, about 5 minutes. Add the tahini mixture and stir until mixed. Bring just to a simmer, and serve immediately.