Crack one egg into a bowl and whisk with a pinch of salt. Set aside.
If using peppers, slice 6 to 8 pieces thinly on a bias. Set aside.
Pat the cod fillet dry with a paper towel and cut into 1/3 inch slices (6 to 8 pieces)
Season with salt, pepper, and garlic powder
Coat each slice in flour
Add neutral oil to a fry pan over medium low heat. One by one, dip the floured cod slices into the whisked egg, then put them in the fry pan. Add the sliced peppers on top if using.
Flip over after 2-3 minutes, or when the bottom is golden brown with crispy edges. Cook for another 2-3 minutes.
Rest on paper towels to collect any excess oil. Serve immediately or store in the fridge for up to 2 days.
Dipping sauce (optional)
Combine soy sauce, water, plum syrup, sesame oil, gochugaru, garlic, green onion, and sesame seeds.