Step 1

Make the spice rub. Place the brown sugar, paprika, garlic powder, black pepper, chipotle pepper, onion powder, and thyme in a small bowl and whisk with a fork to combine. Transfer 2 tablespoons to a small saucepan for the sauce and set aside. Measure out your mustard and salt for the ribs before you start dealing with the raw meat.

Step 2

Remove the membrane from the ribs. Removing this chewy membrane will make for more tender ribs. Flip the racks over so that the bones are facing up. Pry a finger between the silvery membrane and the ribs to start pulling up the membrane — I like to do this between two of the rib bones. Once you have a piece of membrane pulled up, you should be able to easily pull the whole membrane off.

Step 3

Cut the ribs into 4 rib sections. Flip the ribs back over and cut them into 4 to 5 bone sections with a sharp knife. To do this, carefully hold the rack upright and guide the knife through the meat between the bones. This makes them easier to fit into the slow cooker and portions them into individual servings at the same time.

Step 4

Salt the ribs and coat with mustard. Sprinkle the ribs all over with the salt. It's okay if you have some leftover. Using a pastry brush or your fingertips, brush the ribs on all sides with mustard — this helps the spice rub stick.

Step 5

Rub the ribs with the spice rub. Evenly sprinkle the ribs with the spice rub, and then press it into the fleshy part of the ribs.

Step 6

Transfer to the slow cooker and add 1/2 cup water. Transfer the sections of ribs to a 6-quart or larger slow cooker, standing them upright with the meatiest part of the rib down. Add 1/2 cup of water.

Step 7

Cook on low for 6 to 7 hours. Cover and cook the ribs on the LOW setting until the ribs are just becoming tender but not falling off the bone yet, 6 to 7 hours. If you pick up a section of ribs with tongs, it should easily bend but not fall apart. Cover and keep slow cooking while you make the sauce.

Step 8

Make the barbecue sauce. Add the ketchup, brown sugar, vinegar, and liquid smoke if using to the spice rub in the saucepan. Bring to a simmer over medium heat, stirring to combine. Simmer until darkened in color and thickened just a bit, 5 to 7 minutes. Transfer half the sauce to a small bowl — no need to be exact, but it should be about 1 cup.

Step 9

Add 1 cup of barbecue sauce and cook for 2 hours more. Transfer the remaining sauce from the saucepan to the slow cooker, pouring it as evenly over the ribs as possible. Don't worry if they don't seem evenly coated or that there's a bit of liquid in the bottom of the slow cooker. Cover and cook for 2 hours more.

Step 10

Heat broiler to high. When the ribs are very tender (you'll know because picking up the ribs will result in more cracking than bending between the bones), arrange a rack in the middle of the oven and heat the broiler to high. Line a baking sheet with heavy-duty aluminum foil. Transfer the ribs to the baking sheet meaty-side up.

Step 11

Broil ribs for 7 to 8 minutes. Coat the ribs with the reserved sauce. Broil until the sauce is sticky and charred in spots, 7 to 8 minutes.