Bring water to a boil in a large pot. Add the boiled beef, peppercorns, cloves, juniper berries, bay leaves, quartered onion, and quartered tomato. Reduce heat to medium and simmer for 2 hours 30 minutes.
Remove the beef from the broth and let it cool completely. Once cooled, cut it into very thin slices and arrange them on a plate.
Cut the spring onions into thin rings and sprinkle them over the beef slices. In a bowl, whisk together the wine vinegar, oil, salt, and pepper to make the marinade.
Heat the butter in a pan over medium heat. Add the bread cubes and toast them until light brown. Pour the marinade over the beef and spring onions, then scatter the toasted bread cubes on top. Serve immediately.