Scald banana leaf in hot water to soften. Line an 8-inch deep casserole dish with the banana leaves. Trim off excess banana leaf.
Arrange 12 wild betel leaves (daun kaduk) to cover the base of the casserole dish. Place fish steaks on the top of the wild betel leaves (daun kaduk). Sprinkle kaffir lime leaves on top of the fish. Set aside.
Combine custard ingredients in a medium sized bowl.
Beat with a whisk until custard mixture is smooth.
Blend spice paste ingredients in a blender.
Pour spice mix puree into the custard mixture bowl. Stir until well incorporated.
Pour half of the custard and spice mixture over the fish. Arrange remaining 12 wild betel leaves (daun kaduk) on the top and pour remaining custard and spice mixture over the wild betel leaves (daun kaduk).
Fill a large wok or pan with about 2 inches of water. Place a metal rack or stand in the wok or pan. Cover with a lid and bring water to a boil.
When water comes to a boil, transfer casserole dish onto metal rack. Cover and steam over rapidly boiling water for about 15 minutes.
Remove and serve immediately.
** I used salmon steaks but you can use other kinds of white fish steaks.*** Since I do not have wild betel leaves (daun kaduk), I used homegrown shiso leaves as a substitute.