The filling is a paste that's made of shrimp and fish, and while you can sometimes buy this at a Chinese seafood counter, it's good to know how to make it from scratch!
Start by cleaning and peeling the shrimp (10 oz). To peel the shell off, twist and wiggle the tail off. Then, hook a finger under the remaining shell and pull it straight off. If the shrimp is not de-veined already, slide a toothpick or tip of a knife under the dark vein and pull it out.
Give the shrimp a quick wash with clean water to rinse off any debris and shell bits. Squeeze all the water out, and then set it aside.
Prepare the fish fillet (10 oz) by patting it dry with a paper towel. Then, lay it down on your chopping board and cut it into thin slices. Once the fish is all chopped into thin slices, you're going to start smashing it. It's best to use the broadest knife you have for this, the closest thing you have to a cleaver.
Grip your cleaver high on the handle, extend your index finger out to push down on the blade, and turn the cleaver horizontally. Carefully smash the fish slices down into the cutting board. You won't need a lot of sudden, dramatic force, as the fish is delicate and will succumb easily to smashing.
The smashed fish slices should begin to form a paste, and as you go through the fish, gather it back up into new piles to smash again. It should get stickier and more paste-like with the second pass.
Then, return to a normal knife-holding position and give the fish paste a thorough chop. This will severe any remaining long strands of connective tissue and ensure that the fish paste is nice and soft.
Next, start making the shrimp paste. There's no need to slice it first, so proceed directly to smashing. After smashing the shrimp, then chop it thoroughly. Gather the forming shrimp paste together and smash it further to get the paste even finer and stickier.
Next, mix the fish and shrimp pastes together. Fold them together, then give this mixed paste another thorough smashing.
Marinate filling
Now that we have the shrimp and fish paste, we can finally marinate it. Add salt (1 tsp), sugar (1 tsp), white pepper (0.50 tsp), cornstarch (2 tbsp), and water (2 tbsp) to a bowl. Mix it up and pour it over the shrimp and fish paste.
Mix it very, very thoroughly until the paste has absorbed the marinade and become a sticky paste. The more you mix it, the bouncier and chewier your paste will be when you cook it! Add sesame oil (1 tbsp) and mix it in well.
Put the shrimp and fish paste into the refrigerator to chill while you prepare the peppers in the next step.