In a medium bowl, whisk together the finely chopped onion, ketchup, dark brown sugar, cider vinegar, molasses, Dijon mustard, and sambal oelek until the mixture is smooth and well-combined.

Trim any excess fat from the boneless pork shoulder, then place the pieces in the slow cooker. Pour the prepared barbecue sauce over the pork, ensuring it is well coated. Set your slow cooker to High and cook for approximately 6 hours, or until the pork is fork-tender.

Once the cooking time is complete, carefully remove the pork shoulder from the slow cooker and transfer it to a cutting board. Using two forks, shred the pork into bite-sized strands.

Pour the remaining sauce from the slow cooker into a saucepan and place it on medium-high heat. Bring the sauce to a boil, then reduce the heat and let it simmer for about 15 minutes, until it thickens to a glaze-like consistency.

Stir the shredded pork into the thickened barbecue sauce, ensuring each strand is generously coated. Season with kosher salt and freshly ground black pepper to taste.