Preheat the oven to 200°C Heat the oil in a pan and add the onion, carrot and celery and stir so that all the vegetables are coated. Cover and cook over a low heat for 5 minutes, stirring occasionally.
Add the turkey mince and cook over a medium heat, stirring all the time to break up any clumps, until it changes colour. Stir in the flour.
Add the stock, herbs and tomato ketchup and simmer for 5 minutes. Put in an ovenproof dish.
While the meat is cooking, boil the sweet potato until tender. Drain and mash with the butter. Spread over the meat and bake for 15 minutes.