Summarize:

It takes only a few minutes to prepare the roast for cooking, so preheat the oven before you get started. Choose a metal, glass or ceramic pan that's just large enough to hold the entire leg of lamb. If you're roasting the lamb with vegetables, it's fine to use a slightly larger roasting pan. Place the garlic, rosemary leaves, salt, pepper, and lemon zest in a bowl. Pour in the olive oil and mix well. Rub the mixture all over the leg of lamb.  If the mixture seems dry, add a bit more olive oil. You need enough of the rub to cover the entire leg of lamb. Make sure to cover the entire surface of the meat, so that each part cooks evenly and has great flavor. Arrange them around the leg of lamb in a single layer, so that the vegetables cook evenly. As they cook, they'll absorb juices that run off the meat.  Don't pile vegetables up the sides of the leg of lamb, since this may cause the lamb to cook unevenly. If you use russet potatoes, you might want to parboil them first to make sure they cook all the way through. Peel the potatoes and chop them coarsely. Bring a pot of water to a boil and boil the potatoes for 10 minutes, until they are halfway tender. Add them to the roasting pan when the meat is ready to cook.
Preheat the oven to 400 °F (204 °C). Place the leg of lamb in a roasting pan. Rub the lamb with herbs, spices and oil. If you're roasting vegetables, add them to the roasting pan.