In the slow cooker bowl, place the onions, carrots, garlic, herbs, beans, tomato puree and mustard. Season with salt, pepper. Stir to combine all the flavour together.
In a large sauté pan, warm some oil over high heat. Add the shanks. Season well and caramelise for 2-3 minutes until coloured on all sides turning carefully with tongs. Transfer the lamb shanks to the slow cooker.
In the pan, add the mushroom, season well with salt and pepper and sauté for 2 minutes with butter. Add the passata and stock. Bring to simmer for 1 minute or so. Pour the mixture over the lamb shanks in the slow cooker. Cover with the lid and cook on low setting for 9 hours or high setting 4-5 hours or until the meat is tender. Check the seasoning and serve with buttery mash potato.