Pat the beef ribs dry with paper towels. Mix salt, black pepper, smoked paprika, and garlic powder in a small bowl. Rub the seasoning all over the ribs.
Heat olive oil in a skillet over medium-high heat. Sear the ribs for 2 to 3 minutes per side until browned.
Place sliced onions in the bottom of the slow cooker. Arrange the ribs on top.
In a bowl, whisk together BBQ sauce, beef broth, brown sugar, Worcestershire sauce, minced garlic, and apple cider vinegar.
Pour the BBQ sauce mixture over the ribs.
Cover and cook on LOW for 8 hours or HIGH for 5 to 6 hours until the meat becomes tender and easily pulls apart with a fork.
Optional: Transfer ribs to a baking sheet, brush with extra BBQ sauce, and broil for 3 to 5 minutes until sticky and caramelized.
Serve warm with mashed potatoes, cornbread, mac and cheese, or coleslaw.