Set oven to 450 degrees Fahrenheit, and place an oven rack in the middle position. In a large bowl, add the sweet potato, onions, 2 tablespoons of olive oil, cinnamon, thyme, cumin, chili powder, and salt and pepper. Mix until blended evenly. Place mixture on an oven tray, laying sweet potatoes out flat, with onions surrounding it. Bake for 20 to 25 minutes, until potato is fully cooked and the onion is soft. In a small bowl, mix together remaining olive oil and balsamic vinegar. Toss in the greens and whisk lightly together to cover. Spread the goat cheese on the bottom roll, add potatoes and onions on top, then finish with a layer of mixed greens. Top with remaining slice and enjoy!