To prepare falafel, place first 8 ingredients in a food processor; process mixture until smooth. Divide mixture into 16 equal portions, and shape each portion into a 1/4-inch-thick patty. Heat the olive oil in a large nonstick skillet over medium-high heat. Add the patties, and cook 5 minutes on each side or until patties are browned.
To prepare sauce, combine yogurt, lemon juice, tahini, and 1 garlic clove, stirring mixture with a whisk. Spread about 1 1/2 tablespoons tahini sauce into each pita half. Fill each pita half with 1 lettuce leaf, 2 tomato slices, and 2 patties.