Remove the membrane from the concave side of the ribs. Loosen one end with the blunt end of a dinner knife. Use a paper towel to grab the membrane and slowly peel it off. Cut the slabs into individual ribs. Combine 5-spice powder, 2 T. brown sugar, salt, garlic powder and ginger in a small bowl. Toss the ribs in this mixture and set aside.
Mix the the rest of the brown sugar, hoisin sauce, soy sauce, and sherry. Put ribs in a large zipper bag, pour marinade over. Allow to marinate in the refrigerator overnight, turning the bag a couple of times.
Using tongs, remove the ribs from the bag and put in the slow cooker. Reserve marinade in the fridge. Cook on low for 6 or 7 hours.
Preheat the oven to 400°. Cover a sheet pan with foil. Arrange the ribs on the sheet pan. Pour the reserved marinade in a small bowl and add the cornstarch, stirring to make a slurry. Brush the ribs with the marinade slurry and roast the ribs uncovered for 20 minutes or until the edges are getting a little char. Serve hot or at room temperature.