Slow-Cooker Pulled Chicken Sandwich

Ingredients

• 1 medium onion, finely chopped
• 1 can (6 ounces) tomato paste
• 1/4 cup reduced-sodium chicken broth
• 2 tablespoons brown sugar
• 1 tablespoon cider vinegar
• 1 tablespoon yellow mustard
• 1 tablespoon Worcestershire sauce
• 2 garlic cloves, minced
• 2 teaspoons chili powder
• 3/4 teaspoon salt
• 1/8 teaspoon cayenne pepper
• 1-1/2 pounds boneless skinless chicken breasts
• 6 whole wheat hamburger buns, split