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Spray a medium saucepan with a little oil and place on medium heat.
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Cook the onions and celery until they have softened and become translucent - you will need to stir them almost constantly to prevent sticking when cooking with almost no oil.
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Add the garlic, fresh herbs and the tomato purée and cook briefly to release their aromas.
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Add in the carrots and allow them to cook, and meanwhile throw your clean mushrooms into a food processor and blitz until they are a fine, damp confetti (not mush). You can go at them with a sharp knife if you don't have a food processor - just get them as small as you possibly can!
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Tip in the mushrooms and cook for upto 10 minutes which gives the mushrooms time to give up their liquid (which is always more than you think it will be!) and that liquid reduce down to almost nothing.
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Dissolve the stock cube in 500ml boiling water and pour into the pan - bring to the boil then reduce to the simmer and cook, uncovered for 15 minutes. Add in the cornflour which you need to blend with a tbsp of cold water before adding to the pan.
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Meanwhile, pop the prepared celeriac in a pan of boiling salted water and cook until soft - this may take upto 20 minutes. Drain and set aside to let some of the steam off.
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Stir the lentils and peas into the pan of vegetables and transfer to a deep casserole dish.
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Blitz your cooked celeriac with a healthy grating of nutmeg and some salt and pepper with a stick blender (or throw in a food processor) and pulse until very smooth. It never gets smooth enough with a potato masher or ricer.
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Scoop the celeriac purée on top of the pie filling and fluff it up with a fork - use a tall casserole so you don't have to spread the topping too thinly.
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Sprinkle with the reserved thyme and I pop the whole casserole on my digital scale and grate over the parmesan to be sure I don't add too much! Of course, you needn't do this if calories aren't an issue!
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Pop in a preheated oven at 200˚C for about 20 minutes until the top has crisped up, the gravy is oozing out of some places and the pie is piping hot.
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Dig in and eat!