Pat the lamb shanks dry and sprinkle with salt and pepper.
Heat 2 Tbsp of olive oil in cast iron pan over high heat. Sear the lamb shanks until brown all over, about 5 minutes.
Remove lamb from pan and drain excess fat from the pan.
Turn the heat down to medium low. Heat remaining 1 Tbsp of olive oil. Add onion and garlic and 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 tomatoes, tomato paste, thyme and bay leaves. Stir to combine and bring sauce to a simmer.
Place the lamb shanks into the slow cooker and cover with sauce, ensuring shanks are mostly submerged.
Cook on low for 8 hours.
Remove shanks and keep warm. Reduce sauce to desired thickness over stove and/or use a blitz blender to blend the veggies into the sauce.
Serve over potatoes, polenta, or rice. Garnish with fresh thyme leaves.