Asian Chicken Noodle Soup

1 tablespoon Asian sesame oil

1 onion , chopped

2 ribs celery , cut into 1/4-inch slices

4 cloves garlic , smashed

1 1-inch piece fresh ginger , cut into thin slices

2 tablespoons chili powder

1/8 teaspoon dried red- pepper flakes

1/2 quarts canned low-sodium chicken broth or homemade stock

1 cup canned crushed tomato es in thick puree

1/2 pounds boneless, skinless chicken thighs

3 tablespoons Asian fish sauce (nam pla or nuoc mam)

1 cup cilantro leaves plus

1/4 cup chopped cilantro

1 

3/4 teaspoons salt

1/2 pound spaghettini

1/2 head bok choy (about 1 pound), cut crosswise into 1/4-inch slices

1/4 cup lime juice (from about 2 limes)