To make broth: In 6-quart stockpot over medium-high heat, heat oil. Add onion; cook and stir until tender, about 3 minutes. Stir in beef stock, curry paste, soy sauce, brown sugar, garlic powder and ginger. Bring to boil.
Reduce heat to low; cover and simmer 20 minutes. Stir lime juice into broth before serving.
To make Tangy Chili Sauce: In small bowl, mix red chili sauce and lime juice. Cover and refrigerate until ready to serve.
To serve, pour broth into heated 6-quart electric slow cooker or hot pot. Cook steak in broth about 2-3 minutes, or until it reaches desired doneness.
Place lo mein noodles in bowls, then ladle broth and steak over top. Stir in Tangy Chili Sauce and top with mushrooms, baby bok choy and Chinese noodles, if desired.