Roasted Rack of Lamb with Rosemary and Garlic

FOR THE SAUCE:
Melt 2 tablespoons butter in large saucepan over medium heat. Add shallots and saute until tender, about 5 minutes. Add next 3 ingredients. Boil until reduced to 1 2/3 cups, about 40 minutes. Remove from heat. Mix remaining 2 tablespoons butter and flour in small bowl until well blended. Gradually whisk flour mixture into sauce. Simmer until mixture thickens slightly. Keep sauce warm.

FOR THE LAMB:
Preheat oven to 475 degrees F.

Arrange lamb on shallow roasting pans. Blend oil, chopped rosemary and garlic in small bowl. Coat lamb with rosemary mixture. Sprinkle with salt and pepper.

Roast lamb until meat thermometer inserted into center registers 130 degrees F for medium-rare, about 20 minutes. Place lamb on work surface. Let stand 5 minutes. Cut lamb between bones into chops. Arrange lamb on a platter. Garnish with rosemary sprigs and serve. Transfer sauce to sauceboat and serve alongside lamb.

DO-AHEAD TIP:
Sauce can be made 8 hours ahead. Lamb can be coated with rosemary mixture 8 hours ahead. Cover sauce and lamb separately and refrigerate. Rewarm sauce before serving.