Place the sliced onion in the bottom of the slow cooker and set the pork shoulder on top.
Stir together the barbecue sauce, brown sugar, apple cider vinegar, smoked paprika, and minced garlic. Pour the mixture over the pork.
Cover and cook on low for 8 hours until the pork pulls apart easily with a fork.
About 30 minutes before serving, preheat the oven to 375 degrees and grease a small baking dish.
In a bowl, whisk the cornmeal, baking powder, and salt. Add the milk, eggs, and melted butter, then stir in the corn kernels and cheddar cheese until just combined.
Pour the batter into the baking dish and bake for 25 to 30 minutes until the top is golden and the center is set.
Shred the pork in the slow cooker and serve it alongside slices of the warm spoonbread.