Photograph: Jonathan Kennedy
To make the filling, boil the potatoes (in their skins) and celeriac in salted water for 25 minutes or until soft. Peel the potatoes and mash with the celeriac. Leave to cool.
Wilt the spinach in batches, covered, in a large pan over a low heat. Leave to drain over a colander, pressing with a spoon; cool.
Crumble the Wensleydale into generous pieces. Toast the hazelnuts in a dry pan until golden brown; roughly chop.
In a large bowl, mix the potato and celeriac, spinach, hazelnuts, butter, nutmeg, and 1½ teaspoons each of salt and pepper until thoroughly combined, folding in the Wensleydale last so it remains crumbled.
Place one sheet of pastry on a baking sheet lined with nonstick baking paper and brush the edges with beaten egg. Leaving a 4cm border all the way round, pile the filling into the centre and place the second pastry sheet on top. Pinch to seal the edges and brush the whole thing with beaten egg. Trim the edges of the wellington to neaten it, crimp the pastry edges and chill for 1 hour.
For the gravy, heat the oil and butter in a wide pan over a medium heat, add the onions and sugar and cook gently, covered, for 20 minutes or until soft. Turn up the heat and add the wine, stock and bay leaf. Simmer for 20-30 minutes until reduced by half. Discard the bay leaf. Return the liquid to the heat and add the cornflour, whisking to ensure no lumps. Continue to cook for 5 minutes until thick and creamy. Season and keep warm.
Meanwhile preheat the oven to 200°C, fan 180°C, gas 6 and bake the wellington for 35-40 minutes or until golden brown. Serve with the gravy and a green salad.
Get ahead: prepare up to the end of step 5 up to the day before, ready to bake; chill. Bake for an extra 15 minutes