In a medium mixing bowl, combine the finely chopped small onion, ketchup, dark brown sugar, cider vinegar, molasses, Dijon mustard, and sambal oelek. Whisk together until the mixture is smooth and uniform.

Place the pork shoulder in the slow cooker, pour the prepared barbecue sauce mixture evenly over the pork, cover, and set to cook on High for about 6 hours.

Once cooked, transfer the pork to a large cutting board and shred it into bite-sized pieces with two forks.

Pour the remaining liquid into a saucepan and bring to a boil, simmering for approximately 15 minutes until thickened.

Stir the shredded pork back into the thickened sauce, adjust seasoning as needed, and serve hot.