Prepare fish fillets by cutting them into 2-inch pieces.

Lay the fish fillets in a shallow pan.

Sprinkle the fish with chopped celery leaves, salt, and pepper.

Barely cover the fish with water.

Bring the water to a boil and then simmer carefully until the fish flakes easily.

Remove the cooked fish with a slotted spoon and place it in a dish in a warm oven to keep warm.

Strain the fish stock into a clean saucepan, discarding the celery leaves.

In a separate bowl, beat together the egg yolks and whipping cream until well combined.

Stir 1 tablespoon of the hot fish stock into the egg mixture to temper it.

Blend the tempered egg mixture into the remaining fish stock in the saucepan.

Reheat the sauce very slowly, stirring continuously, being careful not to boil.

Adjust the seasoning of the sauce to taste.

Pour the prepared sauce over the fish in the serving dish.

Garnish with chopped parsley before serving.

Serve traditionally by spooning the fish and sauce onto thick slices of dark rye bread.