Preheat The Oven To 350 Degrees F (175 Degrees C). In A Small Bowl, Mix Together The Butter Or Margarine, Mayonnaise, Mustard, Onion, And Poppy Seeds (If Using). Season With Salt And Pepper. Spread A Thin Layer Of The Mixture Over The Cut Sides Of The Buns. Place One Slice Of Cheese On The Bottom Half Of Each Bun, And Top With A Few Slices Of Ham. Wrap Each Sandwich Individually With Aluminum Foil. At This Point, The Sandwiches Could Be Refrigerated Or Frozen In A Plastic Bag. Bake For About 8 Minutes In The Preheated Oven, Or Until Warm All The Way Through. If Sandwiches Are Frozen, Bake For 12 To 15 Minutes.