Heat a large nonstick sauté pan over medium high heat and add about 2 tablespoons of water. Add the mushrooms, sweet potato, garlic, and scallions. Sauté for 5 – 7 minutes or until the mushrooms begin to brown and the sweet potato is just cooked through. Add the spinach, cumin, and cayenne and continue sautéing the vegetables until the spinach is wilted, about 2 – 3 minutes. Remove pan from the heat.