Place the tahini, garlic, salt, olive oil, lemon juice, and water in a medium bowl. Whisk with a hand whisk until smooth.
If the sauce is too thick, add 1-2 tablespoons of water. It should be smooth and creamy. If using, mix in the parsley with a rubber spatula.
Serve immediately, or cover and refrigerate until ready to serve. Stir again before serving, and add more water if the sauce has thickened in the fridge. Add a tablespoon at a time until it reaches your desired consistency.
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Notes
Traditionally, tahini sauce is served as a topping for meat, falafel, or cooked vegetables. It's also delicious with crackers, on top of hard-boiled eggs, or as a salad dressing. You can make it as thin or as thick as you like, depending on how much water you add. A thick sauce is perfect as a dip for vegetables or crackers. A thin one can serve as a salad dressing.
You can keep the leftovers in the fridge, in an airtight container, for up to a week. Stir the sauce again before serving; if it has thickened too much, stir in a bit of water. I don't recommend freezing this sauce.