Over medium heat melt the butter, then reduce to low and add the onion, garlic, and pancetta and cook for 5 minutes, stirring occasionally. Sprinkle with the flour and cook while stirring for 2 to 3 minutes more, until the flour is incorporated well into the butter and coats other ingredients.
Add the hard cider, season to taste with salt and black pepper, and add the bay leaf. Increase the temperature to medium and cook, stirring occasionally, for 15 minutes and the liquid is reduced.
Reduce the heat to low. Place the skinned sole fillets in the pan and let simmer gently, uncovered, for 7 to 8 minutes, then transfer the fillets to a warm serving dish. Remove the bay leaf from the pan.
In a small bowl, mix the lightly beaten egg yolk and heavy cream together. Gradually stir the egg mixture into the still simmering pan juices and heat through until incorporated.
Pour the sauce over the fish in the serving dish. Garnish with a little snipped chives or parsley, and serve with thickly sliced and buttered toasted crusty bread.
Makes 4 servings.
You can use other white fish instead of sole, if you like.