Scrub the potatoes clean, place them in a small saucepan and add enough water to cover them by 2–3 cm. Add the lovage and sea salt and bring to the boil. Boil the potatoes for about 10 minutes, then turn off the heat and leave them to finish cooking until they are just tender.
Slice the onions into 2 mm-thick slices using a sharp knife or a mandoline. Separate the slices so they don’t stick together. Pour the milk and salt into a saucepan, bring to the boil and blanch the onion slices in it for 3 minutes. Drain the onions and leave them to cool. Toss the onion slices in flour, place them in a sieve and shake off any excess flour. Pour the oil into a large, heavy-based pan (4 litres) and heat it to approx. 180 degrees. The oil is ready when it sizzles around a wooden toothpick. Turn the heat down slightly and fry the onions a few at a time in the pan until they are golden brown. Be careful not to add too many onions at once, as this will cause the oil to bubble over.
Remove the onions with a slotted spoon, place them on kitchen paper and leave them to cool. The onions can be prepared 1–2 days before serving. Once they have cooled completely, pack them in an airtight container with a little absorbent paper.
Place all the ingredients into the blender jug at once. Lower the hand blender into the jug so that it touches the bottom. Then blend whilst holding the hand blender at the bottom of the jug. After a short while, you will see that the mayonnaise begins to thicken, and you can now move the blender up and down slightly. Add a little water and blend again to create a creamy homemade herb mayonnaise.
Season to taste with salt and, if desired, extra lemon juice. The mayonnaise tastes best if it’s slightly on the tangy side, and if you’re making it with parsley, season it with a little freshly grated garlic. Homemade herb mayonnaise in approx. 1 min.
Preheat the oven to 100 degrees (not fan-assisted). Place the leaves in the oven until they have wilted slightly (after approx. 8–10 minutes). Wedge a small teaspoon or similar object in the oven door to create a small gap through which the steam can escape. Leave the leaves to dry until the oven has cooled completely (takes approx. 1½–2 hours).
Slice the radishes lengthways into thin slices using a mandoline, soak them briefly in ice-cold water and spin-dry them in a salad spinner. Spread a little butter on the rye bread, cut the potatoes into thick slices or chunks and arrange them on the bread. Dot the potatoes with mayonnaise, then top with the crispy onion rings and thin radish slices. Sprinkle generously with chives and lovage powder.
