Preheat the oven to 100 degrees. Trim the beef from any tendons and fat.
Heat the oil and butter in a frying pan, brown the beef well on all sides and season generously with salt and pepper. Then place the beef in the preheated oven for approx. 45 minutes, until the core temperature reaches 57–58 degrees.
Turn it four times during cooking. Remove the roast from the oven and leave it to cool at room temperature. If it is to be eaten shortly after cooking, or place it in the fridge to cool if it is not to be eaten until the following day.
Slice the onions into 2 mm-thick slices using a sharp knife or a mandoline.
Separate the slices so they don’t stick together. Put the milk and salt in a saucepan, bring to the boil and blanch the onion slices in it for 3 minutes. Drain the onions and leave them to cool.
Coat the onion slices in flour, place them in a sieve and shake off any excess flour.
Heat the oil in a large, heavy-based pan (4 litres) to approx. 180°C.
The oil is ready when it sizzles around a wooden toothpick. Turn the heat down slightly and deep-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 moisture-absorbing paper.
Chop the gherkins roughly in a food processor or with a knife on a chopping board.
Mix them together with the mayonnaise in a bowl.
Peel and scrape the horseradish.
Slice the tenderloin into very thin slices and place or fold the slices onto rye bread spread with butter; top with remoulade, freshly grated horseradish, crispy onions and, if desired, sprigs of chervil.
