Now it's time to cook the rest of the fish. Add the breadcrumbs, salt and pepper to a flat plate and use your fingers to mix them together well.
Have a shallow bowl with the beaten egg ready beside you.
Season the fish filets with salt and pepper, then dip them in the beaten egg and finally press them into the breadcrumbs, making sure they coat the fillets entirely. Place them on a separate plate.
Melt half the amount of butter with half the amount of oil in a 12-inch skillet over medium-high heat. Fry the fish fillets, 2 at a time, for about 3 minutes on each side until they turn golden-brown and the breadcrumb coating becomes nice and crisp.
You will know the fillets are done when the thickest part of the fillet becomes flaky and comes apart easily.
Transfer the cooked fillets to a plate lined with kitchen paper. Remove the leftover grease and any burnt bits from the pan, add the remainder of the butter and oil and fry the remaining 2 fish fillets.