Crockpot Shredded beef

Ingredients
5 tablespoons dark brown sugar, divided
(1-pound) flank steaks
1 cup chopped onion
1 cup tomato paste
4 tablespoons Worcestershire sauce
3 tablespoons molasses
3 tablespoons cider vinegar
1 teaspoon garlic powder
1 teaspoon dry mustard
1/2 teaspoon salt
10 buns, halved

Rub 1 tablespoon brown sugar over both sides of steaks. Combine 1/4 cup brown sugar, onion, and next ingredients (onion through salt) in a crockpot. Add steaks; turn to coat. Cover with lid; cook on high-heat setting for 1 hour. Reduce heat setting to low; cook for 7 hours. Remove steaks; reserve sauce. Shred steaks with 2 forks. Return shredded steak to cooker; stir into sauce. Spoon 1/2 cup steak mixture onto bottom half of each bun.