MIX beer, onion, mustard, vinegar brown sugar, ketchup and salt in medium saucepan until well blended. Bring to boil. Reduce heat to low; simmer 10 minutes or until onion is tender and sauce is slightly thickened. Set aside.
MEANWHILE, brush chicken lightly with oil and season with salt and pepper, if desired. Grill chicken over medium heat 6 to 8 minutes per side or until cooked through, turning occasionally.
SHRED chicken using 2 forks. Add to saucepan with BBQ sauce, tossing to coat well. Serve on rolls topped with coleslaw, if desired.