I cook lamb in two methods, low heat and high heat.
I prep the lamb with slits every inch or inch and a half, then put slices of garlic, sprigs of rosemary and thyme in the every other slit, 1/3 filled with each.
Then cook it either low and slow or hot and fast.
225 - 250 cook it to 140 - 145 internal for medium rare.
425 - 450 cook it till the outside cracks, the inside will be at 140 - 145!
"There is no such thing as a little garlic." A.Baer