Cut the racks of ribs into serving sizes of 2 to 3 ribs. Sprinkle with steak seasoning, and place into a resealable plastic bag; refrigerate overnight.
Preheat oven to 350 degrees F (175 degrees C).
Mix the ketchup, Worcestershire sauce, and honey barbecue sauce in a bowl, and stir to combine. Place the ribs, bone sides down, in a single layer in a large roasting pan. Place slices of onion on the ribs; cover the pan with foil.
Bake in the preheated oven until ribs are cooked through, about 1 hour. Drain off excess fat.
Pick the onion slices from the ribs, and place them into the bottom of a slow cooker. Top the ribs in the roasting pan with the barbecue sauce mixture, reserving 1/2 cup. Pour the reserved half-cup of barbecue sauce mix over the onions in the slow cooker, and set the cooker to Low. Return the ribs to the oven and bake until tender, an additional 45 minutes, basting several times with sauce.
Remove the ribs from the roasting pan and gently place into the slow cooker; pour in sauce. Either serve immediately, or cover the cooker for transporting ribs.
