In large stockpot over high heat, warm chicken broth. Add scallions, garlic, ginger, five spice powder, sugar, soy sauce, fish sauce, Tabasco and sesame oil; bring to a boil. Lower heat to medium-low and simmer 15 minutes.
While broth simmers, place rice noodles in a large bowl and cover with hot water. Soak for 5 minutes, then drain and set aside.
Place a fine-meshed sieve over a large bowl. Strain broth; discard solids that collect. Return strained broth to saucepan and bring to a mild simmer over medium-high heat. Add chicken pieces and cook 3-4 minutes. Add mushrooms; cook another 2 minutes. Add lime juice.
Divide noodles evenly between 4 large soup bowls. Ladle over broth with chicken and mushrooms. Top each bowl of soup with handful of watercress or arugula. Serve with bean sprouts, mint leaves and cilantro leaves.