Yotam Ottolenghi's winter saffron gratin recipe

I've made this a few times using ­different combinations of ­seasonal veg; these four work particularly well together, offering both good texture and hearty sweetness. Still, additions or omissions are ­welcome, so long as you stick to the total weight specified (weights given are after peeling). The veg should be cut into 2mm-thick slices, so use a mandolin if need be; put into acidulated water (ie with lemon juice added) after slicing, to prevent ­discoloration. Serves four. 250g Jerusalem artichokes, peeled and sliced thin 250g swede, peeled and sliced thin 250g kohlrabi, peeled and sliced thin 250g parsnip, peeled and sliced thin 100ml milk 100ml water ½ tsp saffron strands 30g butter, plus extra for greasing 35g plain flour 150ml double cream 60g each chopped parsley and basil 2 tbsp chopped tarragon 60g grated parmesan (it's not vegetarian, so use some other mature cheese, if you'd rather) ¾ tsp salt ¼ tsp white pepper 3 tbsp panko breadcrumbs Preheat the oven to 160C/320F/gas mark 2½. Bring a large pot of water to a boil. Drain the sliced vegetables from their lemon water, add to the pot, blanch for a minute, then drain. Heat the milk and water without boiling, add the saffron and set aside to infuse. Melt the butter in a small pan, add the flour and stir to a paste. Cook gently, stirring, for a few ­minutes, then add the saffron mix and whisk as the liquid thickens. Cook, stirring, for a few minutes, then ­remove from the heat. Add the cream, herbs and half the cheese, season and stir until smooth. Put the drained veg in a bowl, pour the sauce over and use your hands to coat them. Grease a medium ovenproof dish with butter and pour the vegetables and sauce inside; don't bother ­levelling it out too much. Cover with foil and bake for 40 minutes. Remove the foil, mix the remaining cheese with the breadcrumbs and scatter on top. Increase the heat to 190C/375F/gas mark 5 and bake for 15 minutes more, to get a golden-brown crust. Leave to rest for 10 minutes before serving. • Yotam Ottolenghi is chef/patron of Ottolenghi