Preheat Oven To 375 Degrees F (190 Degrees C). Lightly Grease 12 Muffin Tins. Trim Or Cut Bread Into Circles. Place Circles In Bottom Of Muffin Tins. Distribute The Onion And Shredded Cheese Evenly Between The Muffin Tins. In A Medium Bowl, Combine Milk, Eggs, Mustard And Pepper. Divide Between The Muffin Tins. Bake In Preheated Oven For 20 Minutes, Or Until A Toothpick Inserted Into The Center Of A Quiche Comes Out Clean.