Melt butter in a medium saucepan over medium heat until it starts to sizzle.
Add onion and cook until tender. Stir in garlic sauce, cook 1 minute.
Add coconut milk and chicken broth. Cover and cook until boiling.
Uncover broth and add chicken, bell pepper, mushrooms, fish sauce and rice noodles. Cook until it comes to boil.
Continue cooking until the noodles are softened.