Season the pork shoulder chunks with salt and black pepper. In a large skillet, heat the olive oil over medium-high heat. Brown the pork on all sides, about 3-4 minutes per side. Transfer the browned pork to the slow cooker.

In a bowl, whisk together the pineapple juice, diced pineapple, soy sauce, brown sugar, minced garlic, grated ginger, and red pepper flakes (if using). Pour this mixture over the pork in the slow cooker.

Cover and cook on low for 6-8 hours or on high for 3-4 hours, until the pork is tender and easily shreds with a fork.

Once cooked, remove the pork from the slow cooker and set aside. In a small bowl, mix the cornstarch with water to create a slurry. Stir this into the sauce in the slow cooker and cook on high for an additional 15-20 minutes until the sauce thickens.

Shred the pork using two forks and return it to the slow cooker, mixing it with the sauce. Serve over cooked rice or quinoa and garnish with chopped green onions.