In a small bowl combine rosemary, garlic, and olive oil. Rub evenly over the roast. Place the prepared roast on a rack in a shallow roasting pan. Insert a meat thermometer in center of roast. Roast, uncovered, in a 325 degree F oven for 1 to 1-1/2 hours or until the thermometer registers 160 degrees F. Transfer roast to a warm serving platter. Keep roast warm. Reserve 2 tablespoons of the pan drippings.
In a medium saucepan bring 3/4 cup of the apple juice to boiling. Add leek and apple; reduce heat. Cover and simmer for 4 minutes or just until tender. Stir together remaining 1/4 cup apple juice and the cornstarch. Stir into leek mixture. Stir in reserved pan drippings, vinegar, salt, and pepper. Cook and stir until thickened and bubbly. Cook and stir for 2 minutes more.
To serve, cut roast into slices and pass sauce with roast. Makes 6 to 8 servings.