In a small bowl, soak bean-thread noodles in boiling water until soft, about 5 minutes. Drain and cut into 6- to 10-in. lengths.
In a 5- to 6-qt. pot or a 12-in. frying pan (with sides at least 2 in. high), bring broth, mirin, soy sauce, sugar, and ginger to a boil over high heat. Reduce heat and simmer, covered, 5 minutes.
Arrange noodles, chicken, bell pepper, peas, mushrooms, tofu, and green onions in separate piles in the pot. Cover and simmer, without stirring, until chicken chunks are no longer pink in the center (cut one to test), about 5 minutes. Set pot on the table so people can serve themselves, with Sriracha on the side.
Note: Nutritional analysis is per serving.