Peel the garlic clove and crush it with salt in a mortar.
Squeeze the lemon.
Chop the parsley.
In a bowl mix the garlic-salt mixture with the tahini.
Add the juice of half a lemon and mix together. The mixture will now behave somewhat strangely: The tahini reacts with the lemon - and the mixture sticks.
Now add the water and mix everything well with a fork. The sauce is now liquid and creamy.
Now add the chopped parsley and season the sauce again with salt and pepper as required.