Preheat oven to 300°F and place oven rack in the middle.
Mix all rub ingredients together in a sealed large plastic bag.
Cut racks between the bones to make smaller pieces.
Dredge the pieces, in batches, in the sealed plastic bag to cover with spice rub.
Remove pieces and let sit for 20 minutes at room temperature.
Place oil into a large shallow ovenproof heavy-bottomed pot with a tight fitting lid over medium heat.
Once oil has heated, remove lid, add ribs in batches and sear until well browned on both sides.
Remove the ribs and set aside.
Add onion, fennel and celery to the pot, season with salt and pepper and stir, scraping up any browned bits stuck to the bottom of the pot. Cook until the vegetables are tender (about 10 minutes).
Stir in thyme, bay leaf and broth or broth/wine mixture.
Gently place ribs bone side up in the pot and bring the whole mixture to a boil.
Cover the pot and transfer to the oven. Cook until meat falls apart with a fork (about 4 hours)
Place entire mixture into a slow cooker and cook on low setting for 6-8 hours or until fork tender.