Heat olive oil in a large pan over medium-high heat. Brown the lamb shanks on all sides (about 2–3 minutes per side), then transfer them to the slow cooker.
4 lamb shanks, 2 tablespoons olive oil
Add the leek, garlic, celery, carrot, tomato paste, tinned tomatoes, cherry tomatoes, beef stock, and herbs to the slow cooker. Gently stir to combine. Cover and cook on low for 8–9 hours or high for 5–6 hours, until the lamb is tender and falling off the bone.
Once cooked, carefully remove the lamb shanks from the slow cooker and set them aside on a plate, covered to keep warm.
4 lamb shanks
In the final 30–45 minutes of cooking, sauté the sliced mushrooms in a dry or lightly oiled pan until golden. Stir the mushrooms through the sauce in the slow cooker and allow them to simmer and infuse for another 20–30 minutes.
Put back the lamb shanks to the slow cooker just before serving, nestling them back into the rich sauce to warm through.
4 lamb shanks
Serve hot, spooning the sauce and vegetables generously over each lamb shank. Enjoy with creamy mashed potato, soft polenta, or crusty bread.