PREHEAT OVEN TO 375F. Place the whitefish in a baking dish just large enough to hold the fillets comfortably, spread the onions over the top and set aside. Combine mustard, coriander, garlic, wine, vinegar, water, dill, salt and pepper in a small pot. Place over high heat, quickly bring to a boil and pour over the whitefish fillets. Cover the baking dish and place in the oven for 5 minutes. Remove from the oven, let cool to room temperature and place in the refrigerator. Serve chilled.