Preheat the oven to 400ºF. Puncture the sweet potatoes with a fork and place them in a baking dish. Bake until soft, and knife slides into the flesh easily, about 35 – 45 minutes depending on the size.
Meanwhile, prepare the quinoa mixture by heating the oil in a saute pan over medium heat. Saute the garlic and shallot until fragrant, about 3 – 5 minutes. Add the cooked quinoa and hazelnuts, drizzle with lemon juice and season with salt and pepper. Turn down to low and stir occasionally until the sweet potatoes are done.
When the sweet potatoes are soft, remove them from the oven and let them cool for a few minutes. Once cooled, transfer them to a plate, split them open with a sharp knife and spoon the quinoa into the center.