Preheat oven to 350 degrees F.
In a blender, combine the garlic, rosemary, and 3/4 cup of oil; season with salt and pepper. Puree until everything comes together to form a paste; set aside for a few minutes to let the flavors marry.
Lay the lamb out flat, open like a book. Score the surface lightly with a paring knife. Rub the lamb with the garlic paste, being sure to get in the incisions. Roll up the lamb and tie with butcher's twine to hold the roast together.
Prepare an outdoor gas or charcoal grill. Season the lamb generously with salt and pepper, and then place it on the hot grill (put it on the top rack if you are using a gas grill). Sear the outside of the meat, turning, until brown all around, but do not char. Close the grill cover and roast for 30 minutes. The lamb is done when the center is still pink and the internal temperature reads 135 degrees F. Allow the lamb to stand 10 minutes to let the juices settle before cutting off the twine and slicing.
To prepare the pears, halve the pears lengthwise and cut out the cores. Place the pears face down on a baking pan. Drizzle with 4 tablespoons of oil, toss the 8 rosemary sprigs on top, and season with salt and pepper. Bake for 20 to 25 minutes, until the pears are fork tender.
To serve, spoon some polenta on the base of the plate and lay a few slices of the lamb on top; drizzle with a little oil and season with salt and pepper. Put a pear on each plate, face up, and drizzle them with balsamic vinegar. Garnish with a rosemary sprig.
In a large pot, bring the chicken stock and salt to a boil. Gradually whisk in the cornmeal in a slow steady stream. The liquid will be absorbed and the cornmeal will lock up; don't freak, just whisk through it. Lower the heat and continue to whisk until the polenta is thick and smooth, about 20 minutes. Add the cream and butter; continue to stir until incorporated, about 10 minutes.
Remove from heat, fold in the Parmesan and black pepper and serve.