For the filo pastry, lightly oil a large bowl. Sift the flour, paprika and salt into a clean bowl and make a well in the centre. Add the egg and oil then gradually stir in 150ml/5fl oz water, or enough to make a soft dough.
Lightly flour the work surface, turn out the dough and knead for 15 minutes (this dough is best kneaded by pulling it up from the counter and throwing it down - don’t leave it in contact with the counter for too long or it will stick). The dough is ready when it loses its stickiness and is elastic enough to be stretched by approximately 50cm. Place the dough in the oiled bowl, cover with cling film and leave to relax for an hour.
For the leek and potato filling, steam the diced potatoes for approximately five minutes, or until they are cooked through but still hold their shape. Remove from the heat and set aside to cool.
In a frying pan, cook the onions and leeks in a little olive oil until they are soft and the onions are translucent. Add the turmeric, chilli, harissa, potatoes, pistachios and cream to the pan. Remove from the heat, stir in the cheese and breadcrumbs, then season to taste with salt and freshly ground black pepper.
Preheat the oven to 200C/400F/Gas 6, line a baking tray with parchment paper and cover the work surface with a table cloth generously dusted with flour.
Melt the butter and use a rolling pin to roll out the dough to a thickness of 1cm/½in. Stretch the dough by hand taking care not to tear it, pull the dough gently by running your hands under the dough from the centre outwards. The finished dough should be very thin, transparent enough to read through and at least 50cm wide.
Trim the dough edges with scissors to make a neat rectangle then scatter the dough liberally with melted butter from a flicked pastry brush.
Place the cooled filling at one of the narrow ends of the pastry and, using the table cloth to help, gently roll up the strudel. Fold the pastry edges under the strudel and place on the lined baking tray. Curve into a horseshoe shape if it is too big for the tray.
Sprinkle with melted butter and scatter over the paprika and nigella seeds. Bake on the top shelf of the oven for 30-40 minutes or until golden brown.
To serve, blend the coriander with 50ml warmed olive oil in a food processor. Transfer the strudel to a serving plate. Place the cherry tomatoes on a baking tray and brush with oil. Use a blowtorch to char the tomato skin then arrange around the strudel with the rocket, radicchio and coriander oil blend.