In advance, bake sweet potatoes. You can do this in advance by placing them in a crock pot and cooking on high about 2-3 hours or on low 4-6 hours. Or you can microwave them for 8-10 minutes.
Cook quinoa. Bring quinoa and 2 cups water to boil in a saucepan. Cover and cook for 20 minutes (set a timer so it doesn't burn!). Keep covered until use.
Preheat oven to 350 degrees. Cut sweet potatoes in half and scoop out of the skin. Discard skins and place sweet potatoes in a bowl and mash with a fork. Add cooked quinoa, scallions, garlic, cumin, salt, and red pepper. Stir well.
Line a baking sheet with parchment paper. Using wet hands, form cakes by using about 1/3 cup sweet potato mixture and forming a patty. Place cakes on baking sheet and bake for 10-15 minutes until firm and crisp on the edges.
Meanwhile, make the sauce by combining all ingredients in a small bullet or blender. Pulse until cilantro is finley chopped and sauce is smooth. Serve cakes with generous dollops of sauce on top.