1. Using a rigid boning knife, trim off all but 1/4 to 1/2 inch of the fat from the exterior of the roast.
2. Using the knife tip, make 5 slits across the top, each 1/8 inch wide and 1/2 inch deep.
3. Place the roast, fat side up, in a roasting pan.
4. In a small bowl, combine the garlic, thyme, rosemary, the 1 Tbs. salt and the 1 1/2 tsp. pepper.
5. Stir in enough olive oil to make a thick paste.
6. Coat the roast with the garlic-herb paste, pushing some into the slits. Let stand for 30 to 60 minutes.
Preheat an oven to 450°F.
7. Roast the meat for 20 minutes.
8. Reduce the oven temperature to 350°F and continue roasting until an instant-read thermometer inserted into the center of the meat, away from the bone, registers 125° to 130°F for very rare to medium-rare, about 1 hour and 10 minutes more.
9. Cover the roast with aluminum foil if it begins to get too brown.
11. Transfer the roast to a carving board, cover loosely with foil and let rest for 15 to 20 minutes.
12 Pour off most of the fat from the pan and set the pan over medium heat.
13. Add the stock and bring to a boil, stirring to scrape up the browned bits from the pan bottom.
14. Whisk in the demi-glace and cook for 3 to 5 minutes.
15. Season the sauce with salt and pepper and transfer to a sauceboat.
16. Carve the roast and arrange on a warmed platter. Pass the sauce alongside.