Combine the first 7 ingredients in a 13 x 9-inch baking dish coated with cooking spray; toss well. Arrange garlic cloves on top of sweet potato mixture, and bake at 400° for 30 minutes. Remove garlic; thinly slice, and set aside. Set sweet potato mixture aside.
Brush 2 teaspoons olive oil over onion slices and mushroom caps. Sprinkle 1/8 teaspoon salt and 1/8 teaspoon pepper over mushrooms. Place onion and mushrooms on a baking sheet; broil 4 minutes on each side or until tender. Slice mushroom caps into thin strips; set mushrooms and onion slices aside.
Place a medium nonstick skillet over medium-high heat until hot. Add 1 tortilla, and cook 15 seconds on each side. Remove from skillet; set aside, and keep warm. Repeat procedure with remaining tortillas.
Spread 1 1/2 teaspoons miso over each tortilla. Divide the garlic, mushrooms, tomato, arugula, and fontina cheese evenly among tortillas, and roll up. Cut each rolled tortilla in half crosswise.
Combine salad greens and watercress; toss well. Arrange 1 1/2 cups salad mixture on each of 4 plates. Combine vinegar and 1 tablespoon oil; stir well. Drizzle vinegar mixture over salads; top each salad with 2 rolled tortilla halves, 3/4 cup sweet potato mixture, and 2 onion slices.