Season the brisket all over with salt and pepper.
Heat oil in a large oven-safe pot or Dutch oven over medium-high heat.
Sear the brisket until browned on both sides, then remove and set aside.
Add onions to the pot and cook, stirring occasionally, until soft and golden, about 15–20 minutes.
Return the brisket to the pot, placing it on top of the onions. Pour in the broth.
Cover and bake at 300°F for 3 1/2 to 4 hours, until the brisket is fork-tender.
Let rest before slicing against the grain. Serve with braised onions and juices.