Preheat oven to 375°F. Coat a 2-quart casserole with cooking spray and arrange half of the sliced potatoes on the bottom, overlapping. In a medium bowl, combine ham, bell pepper, and scallions. Evenly scatter over potatoes; top with remaining potato slices and sprinkle with cheese. In a small saucepan, melt margarine and blend in flour and mustard. Slowly add milk and continue to stir over medium heat until sauce thickens; mix in black pepper and paprika. Pour sauce evenly over potatoes and ham. Top with bread crumbs.
Cover and bake for 45 minutes; uncover and continue to bake for an additional 30 minutes or until potatoes are fork tender and golden brown.