To make the pastry, sift the flour and salt into a bowl. Add the butter and rub into the flour with your fingertips until the mixture resembles fine breadcrumbs. Add the egg yolk and 15ml/1 tbsp cold water and mix to firm dough. Chill in the fridge for 30 minutes.

Preheat the oven to 200C. Knead the pastry, then roll it out on a lightly floured work surface and use to line a 23cm/9 in loose-based flan tin. Prick the base all over with baking beans.

Bake the pastry case blind for 15 minutes. Remove the paper and beans and bake for a further 10-15 minutes, until the pastry case is crisp.

Meanwhile, melt the butter in a saucepan, add the onions, cover with a tight-fitting lid and cook for 20 minutes, stirring occasionally, until golden.

Reduce the oven temperature to 180C. Spread the pastry case with mustard and top with the onions.