Use this stock to make flavorful Rice Noodles with Chicken, Pork, and Shrimp.
Heat oil in a saucepan over medium heat. Add onion and garlic, and cook until softened, about 4 minutes. Add shrimp shells, bay leaf, and red-pepper flakes, and cook, stirring, until shells are bright pink, about 1 minute. Add fish sauce, and cook for 1 minute. Stir in stock and water. Raise heat to medium-high, and bring to a boil. Reduce heat, and simmer, partially covered, 20 minutes. Pour through a fine sieve; discard solids. Stock can be stored in an airtight container in the refrigerator for up to 3 days or in the freezer for up to 1 month.