Heat oven to 250~. Grease 2-qt casserole. In large skillet, melt margarine. Saute onions until crisp tender. In large bowl, beat eggs; stir in half & half and ham. Pour egg mixtue into skillet with onions; mix well. Cook over medium heat. As mixture begins to set, gently lift cooked portions with spatula so that thin uncooked portions can flow to bottom. Avoid constant stirring. Cook until eggs are thickened throughout; spoon into prepared casserole dish. Pour soup evenly over top. Bake at 250~ for 30 minutes; sprinkle with cheddar cheese and bake 10-15 minutes longer.