Place pepper on a sheet of heavy-duty aluminum foil; broil, about 4 inches from heat source, charring all sides of pepper thoroughly. Carefully remove pepper from oven and place in a brown bag or aluminum foil; close and seal bag or foil. Allow pepper to stand in bag or foil at room temperature for 20 minutes. Peel, core, and seeds can then be easily removed. With kitchen shears or a sharp knife, cut roasted pepper into bite-size pieces. Set aside while preparing the sauce.
In a medium heavy saucepan, melt butter over low heat. Gradually add flour, blending well with a wire whisk. Gradually blend in milk, blending well with the whisk. Increase heat to moderate and cook, stirring constantly, until sauce is thickened and bubbly hot. Season with salt and pepper to taste. Stir in roasted pepper pieces; set aside.
Prepare potatoes and onions for baking. Drain potatoes thoroughly and arrange one half of the potatoes, one half of the onions, and one half of the ham in a 2-quart casserole lightly coated with non-caloric vegetable cooking spray. Evenly spoon half of the sauce over the layer of potatoes, onion, and ham. Spread remaining potatoes, onion, and ham over the sauce in casserole. Spread remaining sauce over the top layer of potatoes, onion, and ham.
Bake, covered, in a preheated moderate oven (350 degrees F.) for 50 minutes. Uncover and continue baking for about 20 to 30 minutes longer or until potatoes are fork tender. Garnish with sprigs of parsley.
Variations: Add a few drops of Worcestershire sauce and/or hot pepper sauce to white sauce mixture.
Combine ¾ cup fine or Japanese-style (Panko) breadcrumbs or cornflake crumbs and 1 tablespoon melted butter or margarine, tossing lightly to mix. Evenly sprinkle crumb mixture over the top of the scalloped potatoes about 7 minutes before the end of the baking time.
Evenly sprinkle top of scalloped potatoes with freshly grated Parmesan or Romano cheese or finely shredded sharp Cheddar or other desired cheese blend about 7 minutes before the end of the baking time.
Add ½ cup chopped green pepper to white sauce.