Boil the potatoes and sweet potato until they are tender. Drain and mash.
Meanwhile heat the oil in a pan and sauté the carrot (diced), celery and leek for 3-4 minutes. Drain the kidney beans and stir into the pan, along with the other beans. Sprinkle in the flour and cook for 2 minutes while stirring. Pour in the stock and stir until the sauce thickens. Add a dash of Worcestershire sauce, season, and simmer for 2 minutes. Pour the mixture into a small baking dish.
Spoon the mash over the bean mixture and spread using a fork. Sprinkle with the grated Cheddar and place under a hot grill until golden and bubbling.
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