In a large skillet, melt butter and sautee onions and potatos until tender. Add vegetables and vegetable broth and cook until vegetables are warmed through. Mix in soup and milk. Heat until bubbling and remove from heat. If you like, a tablespoon or so of flour may be added to boiling mixture to thicken it, depending on personal preference. Pour Mixture into prepared pan. Sprinkle top with shredded cheese.