In medium size sauce pan sautee diced green onion in 2 tbs butter/margarine, once slightly browned add all "but" 2 tbs of butter/margarine .. heat on low ... once melted add flour and stir until blended ...slowing add milk stirring constantly until mixture is a gravy like smooth consistency (you be the judge if more or less milk is needed making sure cream mixture in not too thick )... add bacon bits and parmesan cheese to creamy mixture and stir. Using a 3 qt lidded casserole dish( sprayed with butter flavored non stick cooking spray) , add potatoes, pour cream mixture over potatoes and mix well.
2Place casserole dish w/lid on in a 350 degree oven and bake for 30 minutes ... remove from oven ... stir in remaining 2 tbs of butter/maragarine and velveeta cheese ... bake an additioanl 30-35 minutes until lightly browned and potatoes are soft. Remove from oven let cool a few minutes before serving. Enjoy!
3Note: If potatoes are too creamy/loose add a tablespoon or 2 of breadcrumbs ... stir and return to oven for an additonal 5 minutes.