Trim the leeks of their root and green tops, then slice lengthwise. If you see any grit, wash the leeks and shake them dry. Cut them crosswise into very thin half-ring slices.

In a braiser or heavy saucepan, melt a good knob of butter in olive oil over gentle heat, then add the leeks, along with a generous pinch of salt and a good grind of black pepper. Give the leeks a good mix and let them sweat for a minute or two. Then add a drizzle of water and cover. Let the leeks stew for 10-15 minutes, until they are very soft, stirring from time to time and adding another drizzle of water if things get too dry. Take care not to brown the leeks at all.

When the leeks are soft, uncover and turn the heat up a bit to let any excess moisture cook off. When you just start hearing the pot sizzle, turn off the heat.

Now lay out the sheet of puff pastry on a cutting board or other clean surface. In the center, lay over a thin 'bed' of the braised leeks about the size and shape of the salmon fillet. Lay the salmon on top of its bed, then cover the fillet, top and sides, with the rest of the leeks.

Now take the sides of the puff pastry and fold it over the salmon and leeks to make a neat package, trimming off any excess pastry. You can use the trimmings to cut out decorative shapes to lay on top of the package. Or if you like, you can make a decorative latticework as pictured (see Notes in blog post for detailed instructions).

Transfer the package to a baking sheet. (If not using the latticework top, prick the top here and there.) Beat the egg yolk and milk until smooth and brush the resulting egg wash all over the top of the pastry.

Bake in a hot 200C/400F oven for about 20-25 minutes, until the pastry is puffed up and golden brown all over and the fish is cooked through.

Let the salmon rest and cool off. Salmone in crosta in best served warm but not hot. It is also perfectly lovely at room temperature.