Place potatoes, onions, and carrots in a large saucepan; add about an inch of water. Cover and cook for about 20 minutes, until just crisp-tender. Add peas; cover and turn off heat.
In a large bowl, gently mix chuck with egg, bread crumbs, marjoram, salt, Worcestershire sauce, and milk. Heat oil in large skillet; drop meat mixture by rounded teaspoonfuls into the hot oil; brown quickly on all sides. Remove with slotted spoon and set aside. To the same skillet, add mushrooms; sauté until tender; remove. Drain off excess fat; add soup to skillet with nutmeg, gravy sauce, onion powder, and garlic powder.
Preheat oven to 425°. Arrange peas, onions, mushrooms, and meatballs in a lightly greased 3-quart casserole. Pour in sauce near edge of casserole.
Mash and season the potatoes; arrange around the edge of casserole; brush with a little milk. Bake casserole for about 35 minutes, until potatoes are browned and casserole is bubbly.
Calories per serving: 462