Preheat the oven to 400°F (200°C). Drizzle a little bit olive oil on the baking tray and sprinkle salt. Cut the the potatoes lengthwise and place them, facing down, on the tray. Bake for 20 minutes or until they are tender.
Cook the quinoa: rinse the quinoa under running water (rinsing removes quinoa's natural coating, called saponin, which can make it taste bitter or soapy. Although boxed quinoa is often pre-rinsed, it doesn't hurt to give the seeds an additional rinse at home). Boil 2¼ cups water in a medium sauce pan. Add the quinoa and bring to a boil. Cover the pan and simmer for 12 minutes. Let the quinoa cool down for a few minutes before you use it in the salad.
Meanwhile dice the onion, chop the parsley and set aside.
Prepare the tahini sauce: put the tahini butter in a medium bowl. Squeeze in the lemon juice and stir gently until the tahini begins to thicken and harden. Little by little stir in the water until the texture becomes very smooth and soft. Put it a side.
Put the quinoa into a medium bowl. Add the onion, parsley, olive oil and salt and toss.
Serve 1 or two sweet potatoes halves, facing up, on the plate, cover with quinoa salad but leave the potato ends exposed. Drizzle tahini sauce over, sprinkle some chili pepper and garnish with parsley.