Stir the quinoa in a saucepan over medium heat until it begins to take on a toasty aroma, about 5 minutes. Pour in the water, and add pinch of salt. Bring to a boil, then reduce heat to medium-low, cover, and simmer until the quinoa is tender, about 20 minutes.
Meanwhile, heat the olive oil in a large skillet over medium heat. Stir in the garlic and onion, and cook until the onion has softened and turned translucent, about 5 minutes. Add the mushrooms, sweet potatoes, and cayenne pepper; season to taste with salt and pepper. Cover the skillet, reduce heat to medium-low, and cook until the sweet potato is soft, about 20 minutes, stirring occasionally. Pour a splash of water into the skillet if needed to keep the vegetables from burning. Spoon the vegetable mixture over a bed of quinoa, and sprinkle with chopped pecans to serve.