Preheat the oven to 425F. Place the shallots and wine in a small saucepan and boil hard until the wine is reduced by two-thirds. Stir the mustard into the cream and stir into the shallot mixture. Season with salt and pepper. Pour the butter and lemon juice into a large shallow ovenproof dish. Place the sole fillets in the dish and bake until the fish flakes, about 5 minutes. Pour the cream sauce over the sole and bake for 5 minutes longer.
