Heat oven to 425°. Lightly grease a 2 1/2- to 3-quart baking dish.
In a medium saucepan, melt the butter. Add onion and celery and cook until softened. Stir in flour until blended. Add the chicken broth and cook, stirring, until thickened. Stir in marjoram and salt and pepper, to taste. Add peas and carrots or peas or green beans, chicken, and ham. Spoon into the prepared baking dish. Spoon hot mashed potatoes evenly over the top and spread gently to cover. Put the baking dish on a foil-lined baking pan to catch any drips. Bake for about 20 minutes, until hot and bubbly. To brown the potatoes a little more, turn the broiler on for a minute or two.
Calories per serving: 404