In a small bowl, mix together onion, garlic, paprika, sugar, ginger, mustard, cumin, and cayenne.
Place brisket on a rimmed baking sheet, pat dry with paper towels, and generously season both sides with salt and pepper.
Pat rub onto the brisket on all surfaces. Refrigerate, uncovered (or loosely covered with plastic wrap), for 24-36 hours.
Preheat oven to 350°F.
Allow brisket to come to room temperature for 1 hour.
Lightly coat the bottom of a Dutch oven or baking dish (large enough to hold the brisket and onions) with oil.
Add onions and season with salt and pepper. Place brisket on top of onions.
Roast at 350°F, uncovered, for 30 minutes on each side.
Pour and brush silan over brisket then pour over the broth. Lower the oven temperature to 300°F. Cover brisket and continue cooking at 300°F for about 3 hours or until a fork can be inserted into and removed from the center of the brisket with no resistance.
Carefully remove brisket to a cutting board and let rest for 20 minutes.
Cover loosely with foil if you will be serving immediately.
Slice brisket against the grain and arrange on a platter. Serve with onions and pan juices.
Want recipes like this delivered straight to your inbox? Sign up now for Jamie Geller's Weekly Newsletter.