In a food processor or mortar and pestle, blend together the lemongrass, garlic, shallots, galangal, red chilies, shrimp paste, turmeric powder, ground coriander, and ground cumin until you have a smooth paste.
In a large bowl, mix the amok paste with the coconut milk, fish sauce, and palm sugar.
Add the fish pieces and mix well to coat them with the paste. Let it marinate for about 30 minutes.
Preheat your steamer.
Place a piece of banana leaf or foil in a steaming dish or bowl. If using banana leaves, cut them into squares and fold them into small cups or wrappers.
Spoon a portion of the marinated fish mixture into each wrapper or bowl.
Top with sliced mushrooms and bell peppers.
Place the wrapped or bowl-contained fish mixture into the steamer.
Steam over medium heat for about 20-30 minutes, or until the fish is cooked through and the mixture has set.
Garnish with fresh basil leaves and chopped cilantro (if using).
Enjoy your meal!