Place chicken breasts in the bottom of your slow cooker.
Pour the BBQ sauce and the entire can of pineapple (including juice) over the chicken.
Add chopped onions if using, and season with salt and pepper.
Cover and cook on low for 6-7 hours or high for 3-4 hours, until the chicken is cooked through and tender.
Shred the chicken with two forks inside the crockpot and stir to mix with the sauce.
Serve over cooked rice and garnish with chopped green onions.