In skillet, heat oil over medium-high heat. Brown chicken about 5 min. Remove to plate. In same pan, bring broth, water, ginger, pepper flakes, teriyaki sauce & garlic to boiling.
Add spaghetti, simmer covered 8 mins, stirring often. Stir in vegetables & chicken & bring to a boil. Simmer, covered, 4 mins. Stir in onions, heat 1 min & serve.