First, trim the spareribs: Remove the skirt from the bone side. Remove the breastbone by cutting on a diagonal line just below the widest bones on the end of the rack. Trim off any excess meat on the bottom of the rib rack by rounding out the bottom below the cartilage pieces. Trim the last rib bone off by cutting on the outside of the second to last/smallest bone.
Flip the ribs over so the meat side is facing up. Remove any lean meat on top of the fat sitting on top of the widest bones. Trimming the ribs will make them cook more evenly and will remove anything that may burn up or get too crispy.
For a good tutorial on this, see the LeRoy and Lewis YouTube channel.
In a small mixing bowl, combine the salt, pepper, garlic, and fennel seeds. Mix to combine thoroughly and season the ribs on both sides with the rub.
Prepare a smoker to cook between 250°F and 275°F (120°C and 135°C).
Cook the ribs bone-side down for 2 to 3 hours, keeping an eye on the bones. You're looking for the meat to pull back a bit from the ends of the bones; the meat will also go through a color change, from pale to light red to deep mahogany.
Once they begin to glisten with rendered fat and juices, flip the ribs over so the bone side is up. Cooking the ribs bone-side up will crisp the membrane slightly and cook it so it is edible, not chewy.
The ribs are done when they are tender to the touch between the bones and on the meat below the cartilage. They should be about 190°F to 200°F (90°C).
Prepare a long horizontal piece of foil about 2 times the length of the ribs and another piece of foil perpendicular to that about 2 times the width of the ribs. Place the orange marmalade and butter in the center of the foil and place the ribs meat-side down on top of the marmalade and butter. Wrap them completely in foil and rest for 45 to 60 minutes before carving.
Cut the ribs between the bones, bone-side up, and serve.