Process first 8 ingredients in a food processor until a thick paste forms, stopping to scrape down sides.
Make 3 to 4 (1-inch) cuts in each side of roast with a sharp knife. Fill each cut with about 1 teaspoon of paste. Spread remaining paste over beef. Wrap tightly with plastic wrap; chill at least 8 hours or up to 24 hours. Unwrap roast; place in a 13- x 9-inch pan.
Bake, covered, at 325° for 3 hours and 15 minutes or until meat shreds and a meat thermometer inserted into thickest portion registers 180°. (Meat will not shred below this temperature.) Let stand 10 minutes before shredding or chopping. Garnish, if desired. Serve with corn tortillas, sour cream, Pico de Gallo, and Citrus Onions, if desired.