Cut the ling fillet into 1 1/2-inch/4 cm pieces, place in a bowl and set aside.
Put the white heads of the spring onions (reserving the stalks) and garlic in a mortar and pound to a paste. Add the turmeric, curry powder, yoghurt, fish sauce, sugar, 2 tablespoons oil, and a 1/3 of the dill, roughly chopped, to the fish and mix well.
Cover and marinate in the refrigerator for 1 hour.
Thinly slice 4 of the green spring onion stalks.
Heat a large frying pan over medium heat, add the remaining oil, then fry the fish fillets on one side for 30 seconds.
Turn the fillets over, add the fish stock and simmer for 3 to 5 minutes, until the fish is cooked through. Remove the fish and squeeze over the juice from the lemon.
Mix the bean sprouts, sliced spring onion, remaining dill and vermicelli together.
Place into bowls and spoon over the fish fillets and sauce.