Melt 3 tablespoons butter in a large skillet, add the fish and cook gently for 5 minutes, turning once. Add water, cover and simmer 5 minutes longer. Drain and set aside.
In a large bowl, combine the remaining ingredients (except the corn flake crumbs and cheese); mix well. Fold in the reserved fish, turn into a buttered casserole and top with an even layer of corn flake crumbs. Sprinkle with the cheese; cover and bake at 350 for 25 minutes. Remove the cover and bake 5 minutes longer, until the cheese is golden brown.