Start by filleting the soles, reserving the trimmings for the stock. To prepare the stock, place the trimmings in a saucepan with the onion, bouquet garni, and white wine. Reduce almost to dryness, then add 50 cl water, salt, and pepper. Simmer uncovered on low heat for 30 minutes. Strain through a fine sieve, pressing on the solids to extract maximum flavor.
Preheat the oven to thermostat 6 (180°C).
Peel and finely chop the shallots. Scatter them in the base of a gratin dish, then roll the sole fillets and secure with wooden skewers. Pour over the stock, cover with aluminum foil, and bake for about 10 minutes.
Drain the fillets, transfer to a warm serving dish, and keep covered. Pour the cooking liquid into a saucepan and reduce over high heat. Stir in the cream and mustard, whisking vigorously for 3 minutes on high heat until thickened.
Spoon the sauce over the sole fillets and serve immediately. For an elegant touch, garnish with butter-toasted bread triangles and braised lemon fennel hearts.
