Make the Falafel: Soak the chickpeas in cold water in the refrigerator overnight.
Drain the chickpeas and place them with the onion in the bowl of a food processor. Add the rest of the ingredients, except the oil. Mix well. Process the mixture a second time. Form the mixture into walnut-sized balls and deep-fry or pan-fry in hot oil.
Make the Sandwiches: Stuff the pitas with lettuce and nestle the falafel patties inside. Top with the rest of the ingredients and drizzle with the tahini sauce. Serve immediately.
Mash the garlic and salt together. Add the tahini, mixing well. The sauce will thicken. Gradually add the water, blending thoroughly. Then add the lemon juice. Blend well.
Note: This can be a thin or thick sauce, depending upon the use and preference. Simply adjust with lemon juice and water. This can be used with vegetables or in combination with other recipes.
Recipe courtesy of Madelain Farah, Lebanese Cuisine, Four Walls Eight Windows, 2001