1. Pour stock into a deep cooking pot and bring to a boil.
2. Add the lemongrass and boil for 1-2 minutes. If using fresh lemongrass, also add the tops of the stalks for extra flavor.
3. Add garlic, chili, lime leaves, and mushrooms. Reduce heat to medium and simmer 1-2 minutes.
4. Add the shrimp (plus other vegetables such as cherry tomatoes and broccoli, if using). Simmer 3-4 minutes, or until shrimp is pink and plump.
5. Reduce heat to medium-low and add the coconut milk plus fish sauce and lime juice. Stir well to combine and gently simmer until hot. Test the soup for salt and spice, adding more fish sauce instead of salt ( add 1 more Tbsp.), or more chili as desired. If too sour, add 1/2 tsp. sugar. If too spicy for your taste, add more coconut milk. If too salty, add another squeeze of lime juice.
6. Serve in bowls with fresh coriander sprinkled over. For an extra hit of flavor, you can also add some Thai chili sauce, either store-bought or the homemade version.