Put the tahini butter in a small bowl. Squeeze inside the lemon juice. Stir gently with a spoon until the tahini butter begins to thicken and harden. Gradually stir in some water until the texture is smooth and velvety. Put it a side.
Slice the onion as thin as possible (Japanese mandoline does a great job). Set the onion a side. Dry the parsley leaves with a kitchen towel, then roughly chop them.
Put the roasted sweet potatoes in a medium size bowl, add the onions and parsley, and pour the tahini dressing over. Sprinkle nuts, salt and peppers and serve. You can toss gently and then serve but it looks nicer before tossing.