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Fry the onion and celery in the oil over a low heat until it starts to soften and cook, about 5 mins, then spoon it into the slow cooker. Fry the carrots briefly and add them too.
Brown the sausages all over in the same frying pan – make sure they get a really good colour because they won’t get any browner in the slow cooker. Transfer to the slow cooker and add the sweet potato and tomatoes.
Put the purée in the frying pan and add 250ml boiling water, swirl everything around to pick up every last bit of flavour, and tip the lot into the slow cooker. Add the herbs, stock cube and some pepper. Don’t add salt until the casserole is cooked as the stock can be quite salty. Cook on high for 4 hrs or on low for 8 hrs, then serve or leave to cool and freeze.