Put cellophane noodles in large bowl. Cover with boiling water. Let stand 5 minutes. Drain thoroughly.
Heat oil in wok or deep large skillet over medium high heat. Add onion and carrots and stir fry 3 minutes.
Add garlic and ginger stir fry 30 seconds. Add stock, water and soy sauce. Cover and boil 2 minutes.
Add ham watercress, mushrooms and noodles. Return to boil. Cover, turn off heat and let steep 2 minutes.
Add snow peas, cover and let steep until vegetables are crisp tender about 3 minutes. Stir in sesame oil, rice vinegar and red pepper flakes. Adjust seasoning. Serve in deep bowls, sprinkle with scallions.