Remove lamb from refrigerator and bring to room temperature.
Using a sharp knife, score the fat side of the lamb by making shallow cuts across the lamb. Make 10 to 15 incisions, each about 2 inches deep into the meat. Stuff the holes with sliced garlic, rosemary, and thyme. Drizzle the lamb leg with olive oil, then season generously with salt and pepper.
In a large cast iron pan, heat oil on medium-high heat. Sear all sides of the meat, roughly 2-3 minutes each side. Remove the meat from the pan and turn the heat off.
Add onion, celery, and carrots to the pan. Place the roast on top of the vegetables, fat side up. Place garlic, rosemary, and thyme on the top of the lamb.
Bake at 325˚F for about 1 hour and 15 minutes (135˚F for Med-rare doneness). If the vegetables look dry, add a little water to prevent burning.
Cover meat with tin foil and let rest for 30 minutes, while you make the gravy.
Remove the vegetables. Pour in ½ glass of red wine and let bubble for a minute or two. Sprinkle in flour, add beef stock. Bring to a boil, then simmer until you reach your desired consistency. Using a sieve, remove excess bits from your gravy.