Moisten the loin with 1 teaspoon of olive oil. Combine the rosemary and garlic and rub it into the loin. Season with salt and pepper.
In a large nonstick skillet over medium-high heat, add the remaining oil. Sear the lamb for 8 to 10 minutes on each side, until the meat is firm to the touch and the internal temperature on a meat thermometer is 135 degrees. Remove the meat to a cutting board to rest for 10 minutes.
Meanwhile, wipe the fat from the skillet with a paper towel. Return the skillet to the heat and add the wine. Cook until the liquid has reduced by half, about 1 minute, scraping the pan with a wooden spoon to loosen the browned bits. Add the stock and simmer until the liquid is reduced by half. Meanwhile, dissolve the flour in 1/4 cup of water. Add to the stock and let simmer and thicken, about 1 minute. Season with salt and pepper.
Slice the loin into 1/2-inch slices and arrange among 4 dinner plates. Spoon the pan juices onto each plate and serve alongside Spoon-Mashed Potatoes.