Wash the chick-peas, then soak overnight with the baking soda in cold water to cover. The following day, drain and wash the chick-peas, then cover with fresh water.
Bring to a boil, then lower the heat and simmer until tender, 1-1/2 to 2 hours. Drain and push through sieve or food mill, discarding the coarse fibers remaining in the sieve.
Meanwhile, put the tahini in a small bowl and beat it with the 1/3 cup water until dissolved, using a whisk or fork. Beating briskly with the whisk, add the tahini mixture, a teaspoonful at a time, to the chick-peas, alternating with the lemon juice and olive oil.
When all has been added, crush the garlic over the mixture and sprinkle with optional spices, salt, pepper, and 1 tablespoon chopped parsley. Beat for another minute, then taste for seasonings and chill overnight - if possible. Serve cold, sprinkled with additional chopped parsley.
Makes about 2 cups.