Sift the flour and salt into a large mixing bowl. Add the butter cubes and use your fingertips and thumbs to lightly rub it into the flour until the mixture resembles breadcrumbs. Add the Parmesan (if using) and rub again until the cheese is mixed in evenly. Now add the beaten egg yolk and water, and use a round-bladed knife to combine the wet ingredients with the dry until the pastry comes together. Gather it up with your hands and knead very briefly into a ball on a lightly floured surface. Try not to handle it too much at this stage, or the fat will get warm and the pastry will become tricky to use and may turn out tough and chewy. Wrap in clingfilm and chill for 30 minutes before use.
On a lightly floured work surface, roll out the pastry to about 3mm thick and line your tart tin with it, making sure the edge of the pastry stands just a little proud above the rim of the tin. Trim the edges, then prick the base with a fork and return it to the fridge for 30 minutes. Don’t be tempted to skip this step – it will help prevent shrinkage in the oven.
Preheat the oven to 200°C/fan 180°C/gas mark 6 and put a baking sheet in to heat up. Remove the tart tin from the fridge, line with crumpled non-stick baking paper, fill with baking beans and bake on the hot baking sheet in the oven for 20 minutes. Remove the paper and beans and return the tart tin to the oven for 5–8 minutes or until the base has dried out.
Reduce the oven to 190°C/fan 170°C/gas mark 5. Heat the oil in a frying pan over a medium heat. Cook the onion for 5 minutes to soften, then stir in the spring onions and cook for another minute. Remove to a large bowl and cool. Mix in the smoked chicken, herbs and lemon zest and spoon into your cooked pastry case. Whisk the cream and eggs together and season well with salt and pepper. Pour into the pastry case and scatter the chopped hazelnuts over the top.
Carefully transfer to the hot baking sheet in the oven and bake for 25–30 minutes or until the filling has just set and the pastry is golden brown. Serve warm, scattered with fresh rocket leaves.