Preheat the oven to 425 F. Sprinkle the onions in the bottom of a shallow ovenproof dish. Place the trout on the onions. Season with salt and pepper. Pour in the wine. Place in the oven and as soon as the wine boils, turn off the oven. Cover the dish with foil. Leave the oven door open and let stand for 5 minutes in the oven. Melt the butter in a frying pan. Saute the sorrel until bright green. Season with salt and sprinkle with the cornstarch. Once the sorrel thickens, spoon it into a serving platter and keep warm. Remove the trout from the dish and carefully skin them with a small sharp knife. Keep warm. Reduce the cooking juices from the trout to three-quarters cup. Add half the cream and reduce until thickened. Mix the yolks with the remaining cream and add to the sauce. Heat without boiling, stirring gently. Season with salt and pepper. Place the trout on the platter with the sorrel and spoon over the sauce.
