Preheat the oven to 190C/375F/gas mark 5. Heat the oil in a large saucepan and cook the onion gently until soft and beginning to turn golden. Add the garlic and stir until its wonderful smell wafts up from the pan. Tip in the tomatoes and simmer for about 10 minutes.
Meanwhile, put the lentils in a pan, cover with about 5 cm/2 in of water and simmer for about 20 minutes, until just soft and creamy rather than al dente. Drain if necessary, and then season well with salt and pepper. Stir in the parsley and 3 tablespoons of the cream.
Add the celeriac to the tomato sauce, cover and cook for about 15 minutes or until tender. At first, there will not appear to be enough tomato sauce, but the celeriac will release plenty of moisture as it cooks. When the celeriac is tender, add the remaining cream, the lemon zest and juice, and season with salt and plenty of black pepper to taste.
Layer the celeriac and tomato mixture alternately with the lentils in a large, shallow ovenproof dish, finishing with a layer of celeriac. Sprinkle with the grated Parmesan and bake for about 30 minutes, until nicely browned on the top.
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