Preheat the oven to 180 degrees C. In a small baking dish, stir together stock, garlic and lemon zest. Add the chicken in a single layer and cover with foil. Bake 15 minutes or until chicken is just cooked through. Remove the chicken to a warm plate and cover but reserve the stock.
In a blender, combine 1/4 cup of the stock, the reserved garlic cloves, tahini, lemon juice, yoghurt and salt and pepper to taste. Blend until smooth. Serve the chicken topped with the sauce.