In a pestle and mortar, bash together the garlic cloves, chillis, ginger and coriander stalks until they form a thick paste.
In a big pot, heat the vegetable oil and start sauteing the onions until translucent. Add the sliced red peppers. After around 1 minute, add the garlic, chilli, ginger and coriander paste and saute for 2 minutes until fragrant. Add the coconut milk, salt, pepper and the fish sauce and stir until the coconut milk is heated through.
Add the fish fillets to the sauce, by distributing them in the pot so that they are all covered in the sauce. Bring them to a boil and simmer with the lid on until the fish is cooked through, around 10-15 minutes. You can check a thick piece of fish to make sure.
Finish the dish with sliced spring onions and coriander leaves. Make sure to taste the sauce and adjust the seasoning.
Serve with a side of rice, which you can top with toasted coconut flakes for extra flavor and crunchiness.