Smoky Root Beer Brisket

INGREDIENTS

• 1 whole brisket, 4 to 5 lb., fat trimmed to 1/4 inch
• Kosher salt and freshly ground pepper, to taste
• 1 large red onion, cut into 6 wedges
• 1 large sweet onion, such as Vidalia, cut into 6 wedges
• 1 bunch green onions, white and green portions, cut into 4-inch pieces
• 2 Tbs. olive oil
• 1/2 cup root beer barbecue sauce, plus more for serving