Put the cod, lemon juice and garlic in a large storage bag or shallow bowl and marinate briefly in the refrigerator while you prep and cook the vegetables
Cut up the veggies
Heat olive oil in medium to large pot; add onions and peppers and sauté until the onions are translucent.
Add the potatoes, tomatoes, spices, vegetable broth and fish sauce and bring to a boil; turn heat down to medium to continue boiling for 12-15 minutes or until the potatoes are tender [NOTE: do not add the coconut milk yet]
Add the fish + 1/4 cup of the lemon juice/garlic mixture in the bag or shallow bowl to the pot and boil 5 minutes or until fish is fully cooked
Add the coconut milk; simmer for 5-10 minutes
Taste and add a little salt and pepper to taste + red pepper flakes and/or lemon juice per your preferences
