1.Preheat Oven To 300 Degrees F (150 Degrees C). 2.Combine The Tuna, Mayonnaise, And Mustard In A Medium Bowl, Using A Fork To Mash The Ingredients Together. 3.Top Each Bagel Half With Half Of The Tuna Mixture. Sprinkle The Tops With Shredded Cheddar Cheese. Place Bagel Halves On A Baking Sheet. 4.Bake The Bagel Halves In Preheated Oven Until The Cheese Melts, About 5 Minutes.