Pat the shanks dry and sprinkle with salt and pepper. Heat 2 tbsp of olive oil in a heavy based pot over high heat. Sear the lamb shanks in 2 batches until brown all over, about 5 minutes .Remove lamb onto a plate and drain excess fat (if any) from the pot.
Turn the heat down to medium low. Heat more olive oil in the same pot, if needed.
Add the onion and garlic, cook for 2 minutes. Add carrot and celery. Cook for 5 minutes until onion is translucent and sweet.:
Add the red wine, chicken stock, crushed tomato, tomato paste, thyme and bay leaves. Stir to combine.
Place the lamb shanks into the Slow Cooker. Pour sauce over.
Cook for 5 hours on High.
Check to ensure shanks are ultra tender (use 2 forks) - if not, cover and keep cooking. Ideal is tender meat but still just holding onto bone.
Remove lamb onto plate and keep warm. Pick out and discard bay leaves and thyme.
.Taste then add salt and pepper to taste. Serve hot. Garnish with thyme leaves