Preheat your oven to 450 degrees F.
Start by rinsing the lamb under cold water.
Make a series of deep knife cuts into the top. (see picture for a better idea of what I'm talking about).
Stud the roast with the garlic and rosemary randomly, pushing the pieces in as far as possible. This is going to flavor the meat as it cooks, so you'll want it to "go deep".
Season with salt and pepper, and roast in the oven for 30 minutes.
After 30 minutes, turn the oven down to 350 degrees F.
Roast for around 10-15 per pound (half kilo). So, for a 4lb roast - 40 minutes.
I cook mine to medium, (which is optimal lamb temp) which is about 155 degrees F. If you like your meat a bit more medium-well to well done, roast for an extra 10 minutes or so.
Remove from the oven, and rest tented in foil for about 10 minutes before carving.