Place the beef slices in a large, deep pan with just enough of the beef stock to cover. Bring to a boil, reduce the heat to medium, and simmer for 3 minutes, until the beef is cooked through. Remove the beef slices and keep to one side (and refrigerate until ready to use).
In a bowl, combine the hot beef stock with the cold stock and leave to cool completely. Season with the rice vinegar and soy sauce, then cover and refrigerate until very well chilled.
Bring a large pan of water to a boil, add the noodles, and cook for 4 minutes, until just cooked through, but still a little chewy. Drain and refresh with cold water until the noodles are cold. Drain again and toss with sunflower oil to prevent them from sticking together. Refrigerate until well chilled.
When you are ready to serve, peel and core the pear (if you haven't already done so) and cut into thin julienne strips (don't cut too far ahead of time, as the pear will discolor).
Divide the noodles between 2 bowls, placing the slices of beef, pear, cucumber, and hard-boiled egg on top and scatter over the sesame seeds. Divide the chilled broth between the bowls and serve with the wasabi and some more rice vinegar.
At the table, add in a little of each to taste; it should have a distinct flavor of wasabi. Stir everything together and enjoy.