1.In A 6-Qt. Slow Cooker, Combine The Beef, Green Peppers And Onions. In A Small Bowl, Combine Tomato Paste, Brown Sugar, Vinegar, Chili Powder, Salt, Worcestershire Sauce And Mustard. Stir Into Meat Mixture. Cover And Cook On High For 7-8 Hours Or Until Meat Is Tender. 2.Skim Fat From Cooking Juices. Shred Beef, Using Two Forks. With A Slotted Spoon, Place About 1/2 Cup Beef Mixture On Each Bun.