Season the beef topside joint all over with coarse sea salt and freshly ground black pepper.
In a large saucepan, heat the extra-virgin olive oil over medium-high heat.
When the oil is hot, sear the beef on all sides until well browned, then remove and set aside.
Add the chopped onion, carrot and celery to the pan and cook for 5 minutes until softened.
Return the beef to the pan, add the bay leaves, black peppercorns and coarse sea salt. Cover with water and bring to the boil.
Reduce the heat, cover and simmer for 1 hour 30 minutes, or until the beef is very tender.
Remove the beef and leave it to cool completely in its cooking liquid. Once cool, wrap in cling film and refrigerate for at least 4 hours.
For the tonnata sauce, combine the drained tuna, drained anchovy fillets, capers, mayonnaise and lemon juice in a blender. Blend until smooth.
If the sauce is too thick, gradually add some of the reserved cooking liquid until it reaches a creamy, pourable consistency.
Thinly slice the beef with a sharp knife or a slicer, arranging the slices to overlap slightly on a serving platter.
Spoon the sauce generously over the beef and garnish with a few extra capers.
Refrigerate for 30 minutes before serving to allow the flavours to develop.