Mash the potatoes to a smooth consistency, season with salt and pepper.
Add and knead in the flour and add a drop of milk. Transfer the dough to a lightly floured board and shape into potato cakes about 1 inch thick. In a hot frying pan place a knob of butter, then add the cakes, flip when golden. Turn the heat on the pan down to medium and leave for 4-5 minutes. Serve hot.
For the hollandaise
Put the egg yolks in a sauce pan on low heat. Add the diced butter one knob at a time, whisking all the time. When all the butter has melted you should have a thick creamy consistency, then add the lemon juice and chopped fresh spinach.
Bring a small saucepan of salted water to the boil, reduce the heat and swirl the water. While the water is swirling crack the egg and drop the egg keeping your fingers as close to the water as possible. Leave to cook for 3-4 minutes. Lift out gently on a slotted spoon and drain thoroughly. Finally fry off the lardons on a high heat – enough to crisp them up. To assemble - potato cake, poached egg, topped with hollandaise and scattered with lardoons.