Preheat the oven to 400 degrees F.
Spread the chopped sweet potato over a large baking dish and drizzle enough olive oil to lightly coat, about 2 tablespoons. Sprinkle with sea salt and chili powder. Bake for 30 to 40 minutes, until sweet potato is golden brown and cooked through, stirring once after 20 minutes.
While the sweet potato is roasting, cook the quinoa per package instructions.
Heat black beans on the stove top and toast the corn tortillas.
Prepare tacos by adding desired amount of quinoa, beans, sweet potato, avocado, and green onion to corn tortillas. Drizzle with desired amount of lemon tahini sauce.