Drain and rinse the chickpeas. Place in a stock pot with baking soda and cover with at least 1 inch of cold water. Bring to a boil and simmer until the chickpeas are tender, about 1 1/2 to 4 1/2 hours, depending on the age of your chickpeas. Drain well, reserving 1/4 cup of cooking liquid.
In a food processor, combine chickpeas, salt, and garlic and blend for 30 seconds, periodically stopping the processor to scrape down the sides of the bowl.
Add tahini, lemon juice, and reserved liquid and process 15 seconds. Scrape down the sides of the bowl and continue to process until you have a smooth puree. With the machine running, slowly drizzle in the olive oil and process until incorporated.
Transfer to a serving bowl and drizzle with clarified butter. Top with (or stir in) Za'atar spice if desired.
To roast the garlic, preheat oven to 400 degrees. Cut the top off a bulb of garlic so the cloves are exposed. Set the bulb on a piece of foil large enough to wrap the bulb. Top the exposed cloves with olive oil and wrap the bulb tightly in foil. Bake for 45 to 60 minutes or until a knife slides easily into the middle of the bulb. The garlic can be made up to a week in advance and stored in the refrigerator. Remove cloves from bulb, place in a jar, cover with olive oil, and close lid tightly.
To make the Za'atar spice blend, in a small jar add 1 teaspoon each dried thyme, sesame seeds, and sumac. Add 1/4 teaspoon salt. Shake to combine.
You may substitute two 14- to 16- ounce cans of chickpeas for the dried. Reserve 1/4 cup of the canning liquid for use in Step 3.