Preheat Oven To 375F. Combine wine and mustard in a 3-inch deep baking dish just large enough to hold the whitefish fillets in 1 layer. Place the whitefish in the wine and sprinkle with salt, pepper and thyme. Place the onions and fiddleheads on top, cover the dish and place in the oven for 20 minutes. Remove baking dish from the oven. Arrange a bed of onions and fiddleheads on a platter and place the fish on top. Swirl butter into the cooking liquid and pour over the fish. Serve immediately.
