Preheat your oven to 300°F (150°C) and prepare a roasting pan with a tight-fitting lid or foil.
In a small bowl, combine the sea salt, black pepper, smoked paprika, and garlic powder to create a dry rub.
Pat the beef brisket dry with paper towels and rub the spice mixture evenly over all sides of the meat.
Place the brisket in the roasting pan, cover tightly, and roast for 3 to 4 hours until the meat is tender and easily pierced with a fork.
While the brisket rests after roasting, heat the olive oil in a large skillet over medium-low heat.
Add the thinly sliced yellow onions to the skillet and cook slowly for 20 to 25 minutes, stirring occasionally, until they are deeply golden and sweet.
Stir the balsamic vinegar into the onions during the last minute of cooking to deglaze the pan and add a touch of acidity.
Steam the green beans for 5 to 7 minutes until they are bright green and tender yet still have a slight snap.
Slice the brisket against the grain into thin strips and serve topped with the caramelized onions alongside the steamed green beans.