In a mini food processor with the motor running, add the garlic clove and process until finely chopped. Add the tahini and a pinch of salt. Start the motor and slowly drizzle in the water. Scrape down the sides, and add the lemon juice, yogurt and 1 tablespoon olive oil; process until smooth. Season with salt. The sauce should taste nice and tart (so that it cuts through the richness of the falafel), but if it’s too sour—especially if you’re using fat free yogurt—whiz in another tablespoon of olive oil. If you want a thinner texture, drizzle in some more water as well. Do Ahead: The Tahini Yogurt sauce can be made up to 3 days in advance. Store in a covered container in the refrigerator. Bring to room temperature before serving.