Zest 1 teaspoon of lime zest and set aside. Juice half of the lime (about 1 tablespoon of juice) and reserve the rest for serving.
Heat vegetable oil in a large saucepan over medium heat. Add sliced onion to the pan and cook until it begins to soften, about 4 minutes.
Add curry paste, ginger, and garlic and stir until fragrant, about 2 minutes.
Stir in chicken broth, coconut milk, and fish sauce, scraping up any brown bits. Add mushrooms and lime zest, simmer 15 minutes.
Add bell pepper and rice noodles* (see note), simmer for 3 minutes more.
Stir in raw shrimp or cooked chicken, turn off the heat, and cover. Let rest 5-6 minutes or until shrimp is cooked through or chicken is heated through and the noodles have softened.
Stir in the juiced lime. Serve with fresh herbs and additional lime wedges.
