Rinse fish fillet and then pat dry with paper towels.
Place cornstarch in a large bowl or tray, then dip fillets into cornstarch covering both sides.
Shake off excess until each fillet is covered with a thin dusting of cornstarch, then set aside.
Place oil in the skillet on high heat, then add 1 tbsp of butter.
Carefully place a fillet into the skillet, skin down, and then press down on it with a spatula to stop it from curling up.
Repeat until the skillet is filled with fish.
Cook until skin becomes golden and crispy, flip and cook until the other side becomes golden
You may need to lower heat to medium at times, dependent upon how well your skillet takes on heat.
Repeat instructions #5 to #7 until you have cooked all of the fillets.
Tent fish with aluminum foil to keep warm.
Discard excess oil and butter from skillet then return skillet to a medium heat and add 1 tbsp of butter, ponzu and lemon juice.
Deglaze until ingredients create a sauce then remove from heat.
Divide fillets among four plates and then lightly drizzle with sauce.
Serve with salt and pepper.