Set up your Q for direct cooking. Place the casserole dish (without lid) at the back centre of the Q and add the oil. Preheat on high for ten minutes.
Lightly oil the lamb shanks and season with salt and pepper.
Once the Q is preheated, leave the heat setting on high. Add the onion, carrot and celery to the casserole dish and cook, stirring occasionally, for 5-6 minutes. At the same time add the lamb shanks to the grill around the casserole. Cook for 2½-3 minutes per side, 5-6 minutes in total. Remove the casserole dish from the Q. Set aside the lamb shanks.
Add the flour to the casserole, stir into the mixture, then add the stock and tomatoes and stir well. Place the lamb shanks into the casserole. Finally, add the thyme and rosemary sprigs.
Turn the Q to the roast setting and add a convection tray and trivet using tongs and/or heat proof gloves. Place the lid on the casserole (turning the vent control to steam) and place on the trivet. Cook for 2-2½ hours.
Remove the casserole lid and continue cooking, stirring occasionally, for another 30 minutes, or until the lamb shanks are very tender.