In a pot large enough to fit the hen, bring water to a boil. Add hen, pepper and bay leave and let simmer gently for 1 ½ hour. Transfer hen to cutting board and let cool slightly. When cool enough to handle, pluck the meat from the hen, discarding skin and bones.
Reduce stock by one third. Add root vegetables, leek, and a pinch of salt and boil until vegetables are almost tender. Add the meat. In a small bowl, using a fork, mix cream and sifted flour together until smooth. Add to the pot, stir, and let simmer for 3 more minutes.