Cut the ribs into manageable pieces. Coat them with mustard and liquid smoke, then rub all over with half of the spice mix.
Wrap ribs tightly in foil and marinade overnight.
The next day, bake the ribs wrapped for 2 hours at 170C/340F.
Let the ribs cool down, and pour off the juices (you may add them to the barbecue sauce, or save them for a stew). You may stop here and refrigerate the ribs if making them ahead of time.
Before serving, heat the oven to 200C/400F.
Lay the ribs bone side up on a baking sheet lined with foil and parchment paper. Brush the bone side with barbecue sauce and bake on the top rack for 8-10 minutes.
Flip the ribs over and brush the meat side with sauce and bake for another 8-10 minutes.
Repeat steps 6-7 for 2-3 times. In the meantime, cut potatoes into 1-inch cubes. Microwave on high for 5 minutes twice, stirring in between. Allow potatoes to steam off and cool down.
Fry potatoes in duck fat for 10-20 minutes, stirring occasionally until done to your liking.