Fillet of Sole in Red Wine

Melt 2 Tbsp of butter in a casserole. Add the onion very finely sliced, season with a little salt, and lightly brown. In another casserole, cook the sliced mushrooms in 2 Tbsp of butter and a little salt. Drizzle an oven-dish with 2 Tbsp of butter, line up the sole fillets in it, surround them with the onion and mushrooms, douse with wine and season sparingly with salt and white pepper. Put the pan into the oven, and when cooked (after about 15 minutes), remove the fillets, arrange on an oval serving plate and keep hot. Reduce the cooking liquid by one third, bring to the boil for some minutes, and finish off the sauce away from the heat by adding the remaining butter a little at a time. Arrange the sole fillets in an oven-dish and sprinkle with the red wine sauce. Put the pan into the oven until the fillets are well-browned. Serve hot.