Heat the oil in a medium saucepan over medium heat. Once hot, add the onion, garlic, and ginger and cook until the onion is softened, but not browning, about 5 minutes, stirring occasionally.
Add the coconut milk, coconut aminos, fish sauce, salt, turmeric, and black pepper, and bring up to a gentle boil.
Add the cod, cover the saucepan, and cook until the fish is opaque and flakes easily without a fork, about 5 to 7 minutes.