Bring lots of salted water to a boil in a pot large enough to hold the beef. (If it does not fit, cut the fillet of beef in half.) Add the meat and cover the pot so that the water returns quickly to the boil. When the water is back at a boil, uncover the pot and boil the beef anywhere from 14 to 17 minutes depending upon how rare you like it. While the beef is boiling, in another saucepan cook the potatoes in salted water for 20 minutes or until just tender. In a third shallow saucepan or in a Chinese style bamboo steamer, cook the carrots and leeks, covered, for 10 to 15 minutes or until just tender. To serve, remove the beef from the water and let it rest for 10 minutes. Remove the strings, slice, and serve half of it (reserve half for another dinner) with horseradish garlic mayonnaise and surrounded by half of vegetables (saving half for second time around soup, see below.)