Sprinkle steaks with rosemary and 1 teaspoon salt; season with pepper. Let stand 1 hour.

Heat grill to medium-high; brush grates with oil. Pat steaks dry, then grill until just charred in spots and a thermometer inserted into thickest parts (avoiding bone) registers 125 degrees for medium-rare, 7 to 8 minutes a side. With tongs, sear steaks on edges until lightly charred, about 1 minute each. Transfer to a cutting board; let stand 10 minutes.

Meanwhile, brush both sides of onion rounds with oil. Season with salt and pepper. Grill, turning occasionally, until tender and lightly charred, about 8 minutes. Transfer to a bowl and toss with Worcestershire and vinegar. Season with salt and pepper.

Grill rosemary sprigs until aromatic and just beginning to char, about 10 seconds. Top steaks with sprigs. Slice and serve, with onions.