• Make the croquettes
• Remove the mashed potato from the fridge and shape into 3 or 4 croquettes, about the size of a small sausage.
• Separate your egg- set the yolk aside, and put the white into a bowl.
• Roll the croquettes into the egg white, and then roll in the flour to form a coating.
• Heat some olive oil in a pan- enough to completely cover the bottom of the pan, but you don't need to be drowning in it. Cook the croquettes on medium heat for about 8 minutes, turning regularly to get them evenly brown on all sides.
• Leave them in the pan while you quickly mix up the sauce...
• Make the sauce
• Here's the thing with hollandaise- you want to warm it up enough to taste good, but if you heat it too much the egg yolk will get hard and clump up. So don't put it on direct heat- Boil some water in one pan, and hold a smaller pan over the steam. You could also rest a glass bowl over the pan of water, if you have bowls the right size.
• First put the wine and the egg yolk into the pan and whisk continuously while it heats up. It will start to get a little frothy. Add the olive oil, bit by bit, and keep whisking. Grind in some salt. Whisk some more.
• The sauce will thicken a little, and froth. Once it's the right consistency for you, whisk in a few drops of truffle oil.

Pour the sauce over the croquettes, grab a fork, and dive in.