Preheat the oven to 400°F (200°C)
Pat the leg of lamb dry with paper towels
Trim excess fat if needed
Make small slits all over the lamb
Insert sliced garlic and rosemary into the slits
Rub the lamb with olive oil
Season generously with salt and black pepper
Add thyme, oregano, or other herbs if desired
Place the lamb in a roasting pan
Add onions, carrots, or potatoes around it if desired
Roast at 400°F (200°C) for 20 minutes
Reduce the oven temperature to 325°F (165°C)
Continue roasting until desired doneness is reached
Baste occasionally with pan juices
Check internal temperature with a meat thermometer
Remove at 125°F to 130°F for medium-rare
Remove at 135°F to 140°F for medium
Let the lamb rest for 15 to 20 minutes
Slice against the grain
Serve with pan juices or gravy