Prepare the seasoning rub In a small bowl, mix together the brown sugar, smoked paprika, garlic powder, onion powder, thyme, ground mustard, rosemary, black pepper, salt, cumin, and cayenne (if using).
Pat the ribs dry with paper towels. Rub the seasoning mixture generously all over the ribs, coating all sides. Let sit for 10-30 minutes (if time allows) or overnight.
Arrange the sliced onions so they cover the bottom of the slow cooker pot.
In a medium bowl, whisk together the beef broth, Worcestershire sauce, tomato paste, and liquid smoke (if using). Pour this mixture over the onions. Arrange the seasoned ribs on top of the onions in a single layer (it's okay if they overlap slightly).
Cover and cook on LOW for 6-8 hours or HIGH for 3-4 hours, until the ribs are fork-tender and pull apart easily. Baste the ribs with the cooking liquid halfway through if possible (but not necessary).
Optional: For a caramelized exterior, transfer the cooked ribs to a foil-lined baking sheet. Brush with some of the cooking liquid and broil for 3-5 minutes until the edges are crispy and browned.
Transfer ribs to a serving platter. Strain the cooking liquid if desired, or serve the onions alongside the ribs. Drizzle with some of the cooking liquid. Serve over mashed potatoes or shred the meat and use in pulled pork sliders.
Notes
Ingredient Substitutions Boneless country style pork ribs: Bone-in country style ribs work great (same cooking time). You can also use pork shoulder or pork butt cut into large chunks, though these may need an extra hour of cooking. Onion: Shallots, leeks, or even a handful of pearl onions work well. You can also skip the onions entirely if needed. Brown sugar: Honey, maple syrup, or coconut sugar all work. Use the same amount. Smoked paprika: Regular paprika plus an extra 1/2 teaspoon liquid smoke. Or use regular paprika and skip the smoky flavor. Dried thyme: Fresh thyme (3-4 sprigs) or Italian seasoning (2 teaspoons). Dried rosemary: Fresh rosemary (1 tablespoon chopped) or skip it and add a bit more thyme. Ground mustard: Dijon mustard (1 tablespoon) or yellow mustard (2 teaspoons). Beef broth: Chicken broth, vegetable broth, or even water will work. Beef broth gives the richest flavor. Worcestershire sauce: Soy sauce or balsamic vinegar work in a pinch. Use 1 tablespoon as a substitution. Tomato paste: Ketchup (3 tablespoons) or tomato sauce (1/4 cup). The flavor will be slightly different but still delicious. Liquid smoke: This is completely optional. If you skip it, the smoked paprika will still give you a nice smoky flavor.Cooking Tips Don't skip drying the ribs: Patting the ribs dry helps the seasoning stick better and creates a better texture on the finished ribs. Let the seasoning sit: If you have time, let the seasoned ribs sit for 10-30 minutes at room temperature before cooking. This helps the flavors penetrate the meat. For even better flavor, season them the night before and refrigerate. Layering matters: Placing the onions on the bottom prevents the ribs from sitting directly on the hot surface and helps them cook more evenly. Check for doneness: Ribs are done when they're fork-tender and pull apart easily, about 195º-205ºF. If they're still tough, give them another 30-60 minutes. The onions are gold: Don't skip serving those onions. They absorb all the amazing flavors from the braising liquid and the ribs. Broiling is optional but worth it: Taking those extra 3-5 minutes to broil the ribs gives you crispy, caramelized edges that add great texture. Just watch them closely so they don't burn. Too much liquid? If your slow cooker tends to create a lot of liquid, you can remove the lid during the last 30 minutes of cooking to let some evaporate.Make Ahead Instructions Season the ribs: Rub the seasoning on the ribs up to 24 hours ahead. Cover tightly and refrigerate. This actually gives you better flavor as the seasonings have more time to work into the meat. Prep Ahead: In the evening, slice the onions, season the ribs if you haven't already, and place everything in the slow cooker insert. Cover and refrigerate the entire insert until the morning, or when you're ready to start cooking. Then place the cold insert into the slow cooker and add 30-60 minutes to the cook time since everything is starting cold.Storage Instructions Refrigerator: Store the ribs and any leftover cooking liquid in separate airtight containers in the refrigerator for up to 4 days. The ribs actually taste even better the next day as the flavors continue to develop. Reheating from refrigerator: Place ribs in a baking dish with a splash of the reserved cooking liquid. Cover with foil and warm in a 325°F oven for 20-25 minutes, or until heated through. You can also reheat in the microwave, though the oven method keeps them from drying out. Freezer: These ribs freeze beautifully. Let them cool completely, then place in freezer-safe containers or heavy-duty freezer bags with some of the cooking liquid. Freeze for up to 3 months. Label with the date so you remember when you made them. Reheating from frozen: Thaw overnight in the refrigerator, then reheat using the refrigerator reheating method. For a quicker option, you can reheat from frozen in the slow cooker on LOW for 2-3 hours with a bit of added liquid.A Note About Slow Cooking Times: Slow cookers can vary quite a bit in how they cook. Some run hotter than others, and the age of your slow cooker can affect cooking times too. That's why this recipe gives you a range (6-8 hours on LOW or 3-4 hours on HIGH). Start checking your ribs at the lower end of the time range. They're done when they're fork-tender and pull apart easily. If they're still tough, just give them more time. Older slow cookers tend to run hotter and may cook faster, while newer models with digital controls often cook a bit more slowly and evenly. When in doubt, it's better to cook low and slow. Country style ribs are forgiving and will stay tender even if they cook a bit longer than the suggested time. You really can't overcook them on the LOW setting.