Taiwanese Spicy Beef Noodle Soup

Name: Taiwanese Spicy Beef Noodle Soup
Ingredients: 2 pounds beef stew meat, cut into 1-inch cubes; 3 tablespoons vegetable oil, or more as needed; 8 cups water, or more as needed; 1 (14 ounce) can beef broth; 1 bunch green onions, cut into 2-inch pieces; 1 cup soy sauce; 0.5 cup rice wine; 0.25 cup brown sugar; 10 cloves garlic, peeled, or more to taste; 4 small chile peppers, halved and seeded, or more to taste; 2 tablespoons chile paste, or to taste; 1 (1 1/2 inch) piece fresh ginger, peeled and cut into 5 pieces; 3 star anise pods, or more to taste; 1 teaspoon five-spice powder; 4 small heads baby bok choy; 1 (10 ounce) package udon noodles; 1 tablespoon chopped pickled mustard greens, or to taste