Combine the flour, sugar, yeast and salt in the bowl of a stand mixer fitted with the dough hook.

Warm the milk until lukewarm and pour this into the bowl along with the beaten egg. Mix on low–medium speed to combine, adding 1–2 tablespoons of water, if needed, to bring the dough together. Continue mixing for 2–3 minutes, until the dough is smooth.

Turn out the dough onto your work surface, shape it into a ball and return it to the bowl. Cover and leave it to rise at room temperature for about 1.5 hours, until doubled in size.

Meanwhile, cut the butter into 1cm-thick slices and arrange them side-by-side on a sheet of baking paper. Cover the slices with another sheet of baking paper and flatten the butter into a neat square. Chill for 30 minutes.

Turn out the dough onto a lightly floured work surface. Punch the dough to expel any large pockets of air and roll it out into a neat square.

With the dough positioned squarely, place the chilled butter square at an angle on top. Fold the exposed dough over the butter to completely encase it.

Roll the dough into a neat rectangle three times as long as it is wide. Fold the top third down to cover the middle third and the bottom third up to cover this. Rotate the dough 90 degrees clockwise and chill for 30 minutes.

Repeat the rolling and folding process several times, chilling as needed.

For the topping, melt the butter in a small pan, stir in the flour and cook for 1 minute. Gradually add the milk, stirring until smooth. Bring to a boil, then simmer until thick.

Remove from heat, add mustard and season. Stir in some grated cheese and chill.

Roll the dough into rectangles, cut into squares, and shape into kites. Fill with cheese sauce, top with asparagus, ham, and remaining cheese.

Brush with egg wash and bake at 200°C for about 20 minutes until golden.