Heat 1tbsp of the oil in a large saucepan, tip in the onion and fry for 4-5 minutes until starting to brown. Stir in the diced carrot and swede with the remaining 1/2tbsp oil and fry for 5 minutes. Next stir in the mince, breaking it up with a wooden spoon as you stir and fry until it no longer looks raw. Stir in the flour and cook for a minute or two. Pour in the stock and stir to thicken. Tip in the lentils, tomatoes, tomato purée and Worcestershire sauce. Season with a little salt and pepper then simmer, partly covered with a lid, for 40 minutes until thickened and saucy, stirring occasionally. Taste and season with salt and pepper as needed.
Meanwhile, cook the potatoes for the topping. Cut them into small chunky pieces like mini-roast potatoes and drop them into a pan of boiling water. Simmer for about 9-10 minutes until just tender but still keeping their shape. Drain well in a colander. Spoon the meat layer into a 1.8-2 litre ovenproof pie dish then spoon the potatoes on top and brush with the oil. Season with a little salt and pepper. (Pie can be chilled now for up to a day ahead.)
Preheat the oven to gas 6, 200℃, fan 180℃. Place the pie dish on a baking sheet and bake for about 20-25 minutes or until piping hot and the potatoes are golden. (Or if made ahead and chilled overnight, bake as above allowing 40-45 minutes.) If necessary grill the topping for the last few minutes to brown the potatoes.
After you have assembled the pie and before baking it as above, let it get completely cold, then cover tightly with cling film and foil and freeze (for up to 4 weeks).
To serve, defrost overnight in the fridge then cook as above, for about 40-45 minutes, or until hot and bubbly.
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For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home and view the full Food safety at home video.