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1
Preheat the oven to 180C/gas 4 and bring a pan of slightly salted water to the boil.
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2
Drop the chopped onion into the boiling water.
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3
When it comes back to the boil, drain and set aside the onions.
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4
In the same pan, put the marscarpone, cream, sage leaves, garlic, Parmesan rind (in one chunk) and chilli flakes.
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5
Season heavily as it starts to bubble, then turn the heat right down so that it's just steaming while you prepare everything else.
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6
Peel and halve the celeriac and butternut squash and slice them into 3mm-thick discs.
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7
Take the rind out of the infused cream and discard.
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8
Whizz the cream with a blender until smooth.
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9
Grate the remaining Parmesan and the fontina in a bowl and mix well.
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10
Choose an ovenproof dish big enough to hold all the ingredients and grease it with butter.
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11
Layer it up.
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12
In between each layer of root veg, sprinkle some cheese and blanched onion and spread some of the thick, infused cream.
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13
The top layer should be just onions and cheese.
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14
Cover with foil or a lid.
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15
Cook for 40 minutes-1 hour until a knife can be poked through the veg with relatively little resistance.
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16
Remove the lid or foil, crank the heat up to 200C/gas 6 and give it another 10 minutes or so until the top has gratinated nicely.
