Preheat oven to 450 degrees F (230 degrees C, Gas Mark 8), and heat a large thick-bottomed roasting pan on the stovetop.
Rub the roast generously with salt, then add a little olive oil to the tray and lightly color the meat for a couple of minutes on all sides.
Lay the onions and bulbs of garlic in the tray with the beef on top of them, then cook in the pre-heated oven (about 1 1/2 hours depending on the size of the roast.
After 30 minutes, take the tray out and toss in the vegetables and rosemary. Sprinkle olive oil over the top.
Pick out the garlic bulbs, and carefully squeeze out the garlic, spreading all over the roast.
Grate the ginger over everything in the tray.
Put the roast back in the oven and roast to your taste. I use a digital thermometer, inserting a probe in the centre of the roast and set to my desired setting (135 for medium rare)I just wait for the alarm to sound.
Rare: 120 – 125 degrees F
Medium Rare: 130 – 135 degrees F
Medium: 140 – 145 degrees F
Medium Well: 150 – 155 degrees F
Well Done: 160 degrees F and above
Remove the vegetables to a serving dish to keep warm
Place the beef on a plate, covered with foil, to rest, for half an hour. This lets the juices settle in.