Melt butter in saucepan. Saut mushrooms and onions until tender and mushroom liquid has evaporated. Blend in flour. Gradually stir in milk. Cook and stir over medium heat until mixture comes to a boil and thickens. Add salt and pepper to taste. On bottom half of each croissant layer 2 slices of ham, one quarter of the cheese and one quarter of the mushroom sauce; replace top of croissant. Bake in 180ºC 180C oven about 15 min. or until heated through.