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Heat the oil in a large, deep sauté pan or saucepan. Tip in the onion and thyme sprigs and fry for 2-3 mins. Then add the carrots and fry together for 5-8 mins, stirring occasionally until the vegetables start to brown. Stir in the mince to break it down. Fry for 1-2 mins until no longer pink. Stir in the flour, scraping the bottom of the pan in case the meat sticks, then cook for another 1-2 mins. Pour in the stock and stir until thickened. Stir in the tomatoes, tomato purée, lentils and Worcestershire sauce and season with pepper. Reduce the heat and simmer, covered, for 45 mins, stirring occasionally.
Meanwhile, prepare the topping. Drop all the potatoes into a large pan of boiling water. Bring back to the boil, then simmer for 12-15 mins or until the potatoes are tender. Drain well in a colander then tip back into the pan. Mash with a masher or, briefly, with an electric hand mixer until smooth. Beat in the crème fraîche and milk with a wooden spoon until light and fluffy. Heat oven to 200C/180C fan/gas 6.
Spoon the meat into a 1.5 litre pie dish and remove the thyme sprigs. Top with the mash and smooth over with a knife. Use a fork to create a ridged pattern on top. Place the dish on a baking sheet and bake for about 20-25 mins until piping hot and the filling starts to bubble around the edges. If the top is not brown enough, pop it under the grill for 5 mins or so until the mash is crisp and golden. Let it sit for 5 mins then serve.