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Melt 2 tbsp butter in a large frying pan, chop the remaining butter into small chunks and put in the fridge. Add the leeks, carrots, parsnips and celeriac to the pan with a splash of water, season, cover with a lid and cook for 10-15 mins, stirring now and then, until the veg is beginning to soften.
Stir in 2 tbsp plain flour, then add the stock bit by bit, stirring as you go, until all incorporated and smooth. Cover with a lid and simmer for 20 mins until the vegetables are tender.
Heat oven to 220C/200C fan/gas 6. Add the ham, all but 2 tbsp of the crème fraîche, and the mustard, season with plenty of black pepper and transfer to an ovenproof baking dish. Put the remaining butter and flour in a bowl and season. Rub together with your fingers to a fine crumbly texture. Add the remaining crème fraîche and mix with a cutlery knife until the crumble clumps together in places. Sprinkle the crumble mixture over the filling, then bake for 35-40 mins until crispy and golden on top.