Sprinkle the shallot and the ginger root in a buttered flameproof baking dish large enough to just hold the sole in one layer, arrange the sole in the dish, and season it with the white pepper and salt. Pour the stock over the sole, put a sheet of buttered wax paper directly on the mixture, and bake the sole in a preheated 400° F. oven for 5 to 8 minutes, or until it is opaque and just flakes when tested with a fork. Transfer the sole with a slotted spatula to a platter and keep it warm. Bring the cooking liquid in the baking dish to a boil and boil it until it is reduced by half. Add the cream and boil the mixture until it is thick enough to coat a spoon. Reduce the heat to low and whisk in the butter, 1 piece at a time, adding each new piece before the previous one has melted completely. (The sauce should not get hot enough to liquefy.) Remove the pan from the heat and add the lemon juice, the white pepper, and salt to taste. Blot up any liquid that has accumulated on the platter with paper towels, pour the sauce over the sole, and garnish the sole with the fennel sprigs if desired. Serves 4. FISH 17