Preheat your oven to 300°F (150°C) and prepare a small roasting pan with a piece of heavy-duty aluminum foil.
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.
Place the brisket in the foil, wrap it tightly to seal in the moisture, and roast in the oven 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 and add the thinly sliced yellow onions.
Sauté the onions slowly for about 20 minutes, stirring occasionally, until they are deep golden brown and caramelized.
Stir in the apple cider vinegar to deglaze the pan, scraping up any browned bits, then remove from heat.
Steam the green beans for 5-7 minutes until they are bright green and tender-crisp.
Slice the brisket against the grain and serve topped with the caramelized onions and a side of steamed green beans.