In 3 1/2- to 4-quart slow cooker, place onions and bell pepper. Cut beef brisket across grain into 3 large pieces. Place beef on top of vegetables in slow cooker, overlapping pieces to fit if necessary.
In small bowl, mix barbecue sauce, chili sauce and garlic until blended. Pour over beef.
Cover; cook on Low heat setting 10 to 11 hours.
To serve, remove beef from slow cooker; place on cutting board. With 2 forks, pull beef into shreds. Return beef to slow cooker; stir to mix with sauce. With slotted spoon, serve beef in hoagie buns. Garnish as desired.