Pulled Barbecued Chicken

Makes 6 servings

2 pounds skinless, boneless chicken breasts
1 large onion, chopped
1 can (6 ounces) tomato paste
1/4 cup red or white wine vinegar
1/3 cup packed brown sugar
1 tablespoon Worcestershire sauce
1/4 teaspoon ground chipotle chile
1/2 cup ketchup
1 teaspoon smoked paprika
Seasoned salt and freshly ground black pepper
Toasted sourdough rolls or hamburger buns

Trim any visible fat from the chicken breasts and cut the meat into thin strips. Place the chicken strips in a 5-quart electric slow cooker. Toss with the onion.