Preheat the oven to 400 degrees. Line a baking sheet with aluminum foil or parchment paper and set it aside.
Combine the sweet potatoes with the harissa oil and spices in a large mixing bowl. Mix together until the oil and the spices are evenly distributed over the potatoes.
Spread the seasoned potatoes out onto the sheet pan and bake for 20 minutes. Toss them and turn the sheet pan around so they cook evenly and then cook another 10-15 minutes until the centers are tender and the outsides are crispy.
To make the sauce, combine the parsley, vinegar, tahini and olive oils into a high powered blender and blend together until smooth.