Pat the lamb dry with paper towels. Score the fat by making shallow cuts all over with tip of sharp, small knife. Tuck a sliver of garlic clove inside of each cut. Rub olive oil all over the outside of the roast.
In a small bowl, stir together the minced garlic, sea salt, rosemary and black pepper. Mash it around a bit until it turns into somewhat of a paste.
Place the lamb into lightly oiled roasting pan. Rub paste all over the lamb. MAKE AHEAD TIP: At this point, you can wrap the lamb in plastic wrap and refrigerate for a few hours (or overnight).
Let stand at room temperature for 30 minutes before roasting. Place a meat thermometer into the thickest part of the roast.
Preheat the oven to 425°F.
Roast at the high heat for 15 minutes (uncovered). Then turn the temperature down to 375°F and continue to roast for about another 1 hour to 1½ hours. Watch the meat thermometer closely. When it reaches 150°F., pull the roast out of the oven. Remove the roast to a cutting board and let it rest- the temperature should rise to about 160°F.
MAKE THE SAUCE:
While the lamb is resting, prepare the sauce. Add wine to pan and de-glaze by boiling over moderately high heat, stirring and scraping up brown bits for about 1 minute.
Take out a bit of the liquid and mix with the cornstarch. Add it back to the pan along with the butter. Season with salt and pepper, and serve with the lamb.
Notes
MAKE AHEAD TIP: After placing leg of lamb in roasting pan, you can wrap the lamb in plastic wrap and refrigerate for a few hours (or overnight). You will need to have it sit at room temperature for about an hour before cooking.