Add pork to the slow cooker and top with pineapples and sliced onions.
Whisk together pineapple juice, teriyaki sauce, soy sauce, brown sugar, garlic, ginger, and pepper.
Pour sauce mixture over the pork.
Cook on low for 6–8 hours or on high for 4 hours.
Shred pork with two forks and return to the crockpot.
If sauce needs thickening, mix cornstarch and water, add to the pot, and cook on high for 10 minutes.
Serve warm with your choice of sides.