Boil potatoes for 15 minutes or so until fork tender. Carefully drain using a colander and return the potatoes to the hot pan. Place the pot on the still-hot hob to steam the potatoes dry for a few minutes, shaking occasionally so they don't burn.
Mash with the butter and milk so that it reaches the desired consistency. Set aside to cool while you prepare the filling.
Heat the butter and oil in a saute pan, and gently fry the onion, celery and carrot for 5-10 minutes until it begins to soften.
Add the garlic and cook a further minute or so before addding the lean lamb mince. Saute until browned.
Add the thyme, tomato puree, Worcestershire sauce and lamb stock (from a stock cube is fine) and simmer for 10 minutes until the gravy has thickened.
Arrange the lamb mixture on the bottom of a casserole dish, top with a layer of frozen peas and pipe or spoon the mash over the top.
Dot a little butter over the top of the mashed potatoes, sprinkle with some paprika, if desired, and bake in the centre of the oven for 30 minutes.
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