Melt the butter in a heavy bottomed pan and add the olive oil. Add the onions and fry gently for 10-15 minutes so that they gently brown and melt.
Add the garlic, cider, bay leaves and thyme and simmer for 2-3 minutes. Gently lay the Plaice fillets in the poaching liquor and simmer for a further 6-8 minutes. Lift the fish fillets out of the sauce and place them on a serving plate. Turn the heat up and reduce the sauce by a third. Season to taste.
Spoon some onions on top of each piece of fish and pour the remaining juices around. Alternatively, put the fish back into the pan and take the whole thing to the table. Sprinkle with chopped parsley before serving.