Mix quinoa, water and salt in a saucepan. 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. Add water into the skillet as needed to keep the vegetables from drying out and sticking.
Divide the quinoa among 4 plates and ladle the vegetables over the quinoa. Sprinkle with almond slivers.