Cut the rack in half (or thirds if you have a small slow cooker).
In a small bowl, whisk together the mustard and apple cider vinegar.
Brush the ribs on all sides with the mustard mixture.
Liberally season the ribs on all sides with bbq seasoning.
Add the ribs to the slow cooker. Arrange in a single layer if you can.
Cook on high for 3-4 hours or low for 6-7 hours. The ribs are done when they reach 190 to 200 degrees.
Preheat the oven to broil.
Remove the ribs from the slow cooker and transfer to a baking sheet.
Season with a little more bbq seasoning. Generously brush bbq sauce on top of the ribs.
Broil for 3-4 minutes until the bbq sauce starts to bubble and caramelize.
Remove from the oven and top with a little more bbq sauce. Broil for another 2-3 minutes until the bbq sauce starts to bubble again.
Cut the rib rack into individual ribs. Serve with extra bbq sauce.