Peel and roughly slice the onions, and trim and halve the leeks lengthways.
Heat the olive oil in a flameproof casserole pan over a high heat and cook the onions for 15 minutes, until browned. Deglaze with the white wine, then pour in the vegetable stock. Add the leeks and cook for 30 minutes.
Tie the beef with kitchen string at 2cm intervals, then attach it to each end of a wooden spoon to create a handle – the string should be just long enough so that the spoon can be balanced on top of the pan while the beef is submerged in the stock.
Lower the beef into the pan and poach over a low-medium heat for 15 minutes for rare (20 for medium, 25 for well done).
Take the pan off the heat and transfer the beef to a warm plate. Cover with foil and allow to rest for 5 to 10 minutes. Drain the leeks and keep them warm.
Peel and finely chop the shallot, and finely chop the cornichons. Pick and finely chop the herbs.
In a bowl, combine the mustard, vinegar, savora sauce and rapeseed oil, then add the shallot, cornichons, capers, and herbs, then season to taste.
Slice the beef very thinly, drizzle with the vinaigrette and serve with the leeks on the side.
Savora, or sauce saveur Amora, is a French mustard sauce, slightly sweetened with honey. Find it at French supermarkets or online at fairfayre.com. Alternatively, add 1 teaspoon of honey to the Dijon mustard.