Prepare the rub by mixing kosher salt, black pepper, smoked paprika, garlic powder, and onion powder in a small bowl.
Pat brisket dry and rub the spice mixture evenly over it. Wrap tightly in plastic and refrigerate for at least 12 hours or overnight for best results.
Preheat oven to 300°F (150°C). Heat olive oil in a Dutch oven or roasting pan over medium-high heat. Sear brisket fat-side down for 4-5 minutes per side until a deep brown crust forms. Remove brisket and set aside.
Add sliced onions to the pan and cook 5-7 minutes until softened. Add smashed garlic and cook 1 more minute until fragrant. Pour in beef broth, scraping the pan to lift browned bits.
Place brisket back in the pan, fat-side up, on top of onions. Liquid should come halfway up the brisket sides.
Cover tightly with foil or pan lid and roast in the oven for 4–4.5 hours, until fork-tender and thickest part reaches 200–205°F.
Remove brisket and let rest on a cutting board for at least 30 minutes.
Slice against the grain into ¼-inch thick slices. Serve with pan juices and onions spooned over the top.
Notes
Look for a flat cut brisket with even marbling and fat cap. Resting ensures juicy slices. Pan juices can be strained and simmered into a rich gravy.