Step 2: Smear margarine or butter on the bottom and sides of a deep baking dish and lay fish in the bottom. Sprinkle with salt and pepper and pour milk over all.
Step 3: Cover dish with foil and bake on the middle oven rack for 40 minutes. If you are not making the sauce, fish is now ready to serve.
Step 1: When fish is cooked, remove the pan from the oven. With a spoon or baster, draw off all the liquid into a small saucepan. (Turn down the oven to 200°F and return fish to oven to keep warm.)
Step 2: Bring liquid in the saucepan to a boil over moderate heat.
Step 3: In a cup, mix cornstarch and milk together to make a smooth, thin paste. Gradually pour cornstarch paste into boiling liquid, stirring constantly to prevent lumps. Add parsley and lemon juice. Pour sauce over fish and serve immediately.