In a big bowl, mix the crackers and parmesan. Add salt and pepper. Pour milk in a bowl not too deep. Soak fish filets in milk, then in the cracker mixture. In a skillet, heat oil and butter at medium-high heat. Add fish filets and cook for about 3 minutes on each side or until that the flesh is opaque and is easy to brake with a fork.
In the meantine, in a casserole, bring to boil the envelop of white wine sauce and water, whisking it. Reduce heat to medium-low and cook 1 to 2 minutes or until sauce has thicken lightly. Add Dijon mustard. When ready to serve, top the fish fillets with the sauce.