Place the sliced onion and minced garlic in the bottom of the slow cooker.
Set the pork shoulder on top of the onions. Sprinkle it with salt and black pepper.
Stir together the maple syrup, Dijon mustard, and chicken broth in a small bowl. Pour the mixture over the pork.
Arrange the sweet potato chunks around the pork in the slow cooker.
Cover and cook on low for 8 hours until the pork pulls apart easily.
Remove the sweet potatoes to a bowl. Add the butter and milk, then mash until smooth.
Shred the pork in the slow cooker with two forks and stir it into the remaining sauce.