If making the pastry by hand, put the flour, ½ teaspoon of sea salt and the sugar in a large bowl, then coarsely grate in the butter. Rub the flour and butter with your fingers until it resembles breadcrumbs, but don’t overwork the mixture – the odd lump is fine.
To make the pastry with a food processor, put the flour, ½ teaspoon of sea salt, the sugar and the butter pieces into the food processor and pulse the mixture until it resembles breadcrumbs – it’s important that you pulse so that the spinning motion of the blade does not melt and overwork the butter. Empty the mixture into a large bowl.
Add 70ml of cold water, a splash at a time, mixing until the dough comes together into a firm ball. Cover and refrigerate for at least 30 minutes.
Preheat the oven to 180C/gas 4. Lightly grease a 23cm tart case with butter and dust with flour, tapping off any excess.
Remove the pastry from the fridge and let it rest at room temperature for a few minutes – this will make it easier to work with.
Dust a work surface with flour and roll out the dough, turning it often, and keeping enough flour on the work surface to prevent things sticking. Keep rolling and turning until the pastry is about 3mm thick all over.
Roll the pastry loosely around the rolling pin and gently unroll over the tart case. Using your index finger, push the pastry evenly around the rim, pressing lightly where the edge meets the base. Pinch off any excess pastry. Pop the tin in the fridge for 5 to 10 minutes before baking.
Remove from the fridge, prick the pastry with a fork and line the pastry with greaseproof paper. Fill with dried beans or baking beads and bake for 15 to 20 minutes, then remove the beans and paper and return to the oven for another 5 minutes, or until the pastry is an even light golden brown colour. While the pastry is cooking, make a start on the fillings.
For the filling, slice the cheese, and trim and slice the spring onions into 1cm pieces. Toast the pine nuts. Combine the eggs, yolks and cream in a bowl with a pinch of salt and pepper.
Spread the mustard over the base of the tart, add the sliced cheese and scatter the spring onions on top. Sprinkle with the pine nuts, then pour over the egg mixture.
Turn the oven down to 170C, then pop in the tarts for 30 to 35 minutes or until golden and the contents are set. Serve warm or cold.
Find your nearest stockist of Gubbeen cheese at Gubbeen.com. Alternatively, if you can;r get hold of it, port salut is a good substitute.