Black olive paste gives a rich flavor to the sauce. Longer marination time will help as well.
Pour olive oil into a shallow dish, and distribute half the garlic and one-third of the rosemary in the oil. Slice the lamb across into eight rounds, lightly pound each into a 1/4-inch-thick medallion, and place in oil. Sprinkle the lamb with one-third of the chopped rosemary and the remaining half of garlic; set aside for 20 minutes. (For more intense flavor, marinate several hours or overnight.)
Pour 2 tablespoons oil-garlic marinade into large skillet; place over medium-high heat. Add medallions; cook until browned on the outside, medium rare inside, about 1 minute on each side. Transfer to a platter, cover with foil to keep warm, and pour off the fat from skillet.
Add wine and stock; cook, stirring constantly to loosen any cooked-on browned bits. Add remaining third of rosemary and the olive paste, and cook until the liquid has reduced by two-thirds. Swirl in the butter until the sauce has thickened slightly, about 1 minute. Arrange the medallions on a serving platter, pour the sauce over the lamb, and serve immediately.
