In a medium bowl, whisk together the yogurt, tahini, and lemon juice until smooth.
Stir in the fresh mint, crushed garlic, salt, and ground cumin. Adjust seasonings and lemon juice to taste. If the sauce is too thick, whisk in cold water a teaspoon at a time until you reach your preferred consistency.
Let the sauce chill in the refrigerator for at least 20 minutes before serving. Garnish with additional fresh herbs.