Preheat your oven to 300°F (150°C).
In a small bowl, combine the smoked paprika, garlic powder, sea salt, and black pepper.
Rub the spice blend thoroughly over all sides of the beef brisket to ensure a flavorful crust.
Place the brisket in a roasting pan, cover tightly with foil, and bake for 3 to 4 hours until the meat is fork-tender.
While the brisket rests, heat the olive oil in a skillet over medium-low heat.
Add the sliced yellow onions to the skillet and cook slowly for about 20 minutes, stirring occasionally, until they are golden and caramelized.
Stir in the apple cider vinegar to deglaze the pan, scraping up any browned bits.
Add the fresh spinach to the skillet and sauté for 1-2 minutes until just wilted.
Slice the brisket against the grain and serve topped with the caramelized onions alongside the sautéed spinach.