For Cheese Sauce, In A Saucepan, Melt Butter. Stir In Flour Until Smooth; Gradually Add The Milk. Bring To A Boil; Cook And Stir For 2 Minutes Or Until Thickened. Reduce Heat To Medium-Low. Add The Cheese, Mustard, Salt And Pepper, Stirring Until Cheese Is Melted. Cover And Keep Warm. In A Skillet With High Sides, Bring 2-3 In. Of Water And Vinegar To A Boil. Reduce Heat; Simmer Gently. For Each Egg, Break The Cold Eggs Into A Custard Cup Or Saucer, The Hold The Dish Close To The Water Surface, Slip The Egg Into The Water. Cook 4 Eggs At A Time, Uncovered, For 3-5 Minutes Or Until The Whites Are Completely Set And The Yolks Begin To Thicken. With A Slotted Spoon, Remove Each Egg. Repeat With Remaining Eggs. To Assemble, Top Each Muffin Half With One Slice Canadian Bacon, One Egg, Cheese Sauce And Bacon.