Heat 2 tablespoons of the oil in a large saucepan and fry the onions for 6-8 minutes until softened and starting to brown. Spoon in another 1 tablespoon of oil, then add the carrots, celery and garlic and fry for 4-5 minutes. Tip in 425ml of the stock, the tomatoes, mushrooms, lentils, tomato purée, oregano and paprika, bring to a boil then lower the heat and simmer gently, part covered with a lid, for 20-25 minutes (adding the rest of the stock if needed to cook the lentils) until the lentils and carrots are just cooked and the liquid is absorbed to give a thick, juicy sauce. (If cooked too long the lentils can break down and turn mushy.) If too much stock has been absorbed, stir in a splash more stock.
Meanwhile, put the spinach in a large heatproof bowl. Pour boiling water over so the leaves wilt, then immediately drain in a colander and put under the cold tap. Squeeze out as much moisture as you can with your hands, then chop the spinach and stir it into the cooked lentil mixture. Season to taste with salt and pepper.
Meanwhile, chop the potatoes into 5-6cm chunks and cook in boiling water for 12-15 minutes or until tender. Preheat the oven to Gas Mark 6, 200℃, fan 180℃. Drain the potatoes well in a colander and return to the pan to dry out briefly over a very low heat. Put in the butter and milk and mash until smooth and creamy. Using a wooden spoon, beat in all but a handful of the cheese, the chives and season to taste with salt and pepper. Spoon the lentil mix into 4 individual ovenproof dishes, about 600ml (or use one large dish). Spoon over the mash, spread it out and use a fork to make a rough pattern, then scatter over the rest of the cheese. (Pies can be chilled now for up to a day ahead.) Place the dishes on a baking sheet, then bake for about 25 minutes, or until the topping is golden and the filling bubbly. (Or if made ahead and chilled overnight, bake as above allowing 40-45 minutes.)
Freezing and defrosting guidelines
After you have assembled the pie and before baking it, leave the dish to cool completely, transfer it to an airtight, freezer-safe container, seal and freeze for up to 1-3 months. To serve, defrost thoroughly in the fridge overnight before reheating. Loosely cover with foil and bake until dish is thoroughly heated through. Reheat until piping hot.
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