Preheat the oven to 400°F (200°C). Line a baking pan with baking paper.
Wash the potatoes and scrub their skin. Dry with a clean kitchen towel and cut them into 1" x 1" dice. Pour the oil onto the baking paper and add the potatoes. Mix the potatoes with the oil, sprinkle salt over and put the baking pan into the oven for 20 minutes or until the sweet potatoes are tender.
Meanwhile prepare the tahini. Put the tahini in a small bowl. Squeeze in lemon juice and mix gently with a spoon. The tahini would begin to thicken. Gradually stir in the water. The tahini is ready when the texture is smooth and velvety. Set it aside.
Slice the onion as thin as possible (Japanese mandoline does a great job). Dry the parsley leaves with a kitchen towel and roughly chop them.
Put the roasted sweet potatoes in a medium size bowl, add sliced onions and chopped parsley. Pour the tahini dressing over. Sprinkle nuts and season with salt and peppers. If serving immediately to people you want to impress, then toss at the table. Other wise toss and keep in the fridge, up to 1 day. If you roasted the onions, you can keep in the fridge up to 3 days.