In a small bowl, combine the smoked paprika, garlic powder, sea salt, and black pepper to create a dry rub.
Pat the brisket dry with a paper towel and rub the spice mixture evenly over all sides of the meat.
Heat half of the olive oil in a large skillet over medium-high heat and sear the brisket for 3-4 minutes per side until a golden-brown crust forms.
Transfer the seared brisket to a slow cooker and pour in the beef broth and apple cider vinegar.
Cover and cook on low for 8 hours or on high for 4 hours until the beef is fork-tender.
While the brisket is finishing, thinly slice the yellow onion.
Heat the remaining olive oil in a skillet over medium-low heat; add the onions and cook slowly for 20-25 minutes, stirring occasionally, until they are soft and caramelized.
Remove the brisket from the slow cooker and let it rest for 10 minutes before slicing thinly against the grain.
Serve the sliced brisket topped with a generous portion of the sweet caramelized onions.