1In 9 x 13" glass dish, place cod fillets, minced onion, salt, pepper and water. Cook in microwave approximately 10 to 15 minutes, or until fish flakes easily with a fork.I prefer to use the regular oven so cook for approx 20 mins till fish is partially cooked.
Meanwhile, make white sauce: Place butter and flour in small saucepan. Whisk together over medium heat. Add milk slowly, stirring until mixture thickens.
Pour water off fish, add white sauce and cover with bread crumbs and grated cheese. Bake in preheated 350ºC oven until cheese is melted and golden brown.