Remove heads and entrails from ikan bilis. Wash and soak for 10 minutes. Deep-fry until crisp. Drain and set aside.
Blend 2 tbsp fermented durian and pati santan together with the ground ingredients in a liquidiser.
Heat oil in a wok and stir-fry ground ingredients and lemon grass until fragrant. Add the rest of the fermented durian and bring to a gentle boil over low heat until gravy is thick.
Add salt to taste. Dish out and serve with crisp ikan bilis, petai and rice.