To make the dressing: In a small bowl, combine the tahini, lemon juice, olive oil, garlic, mustard, and salt. Whisk until blended, then add the water and whisk until you have a smooth, blended tahini sauce. Season generously with black pepper. The dressing should be tangy, like classic Caesar dressings. If it's overwhelmingly tart or too thick to pour, whisk in an additional tablespoon of water.