Peel the marrow and cut in half lengthways. Scoop out centre seeds, cut the marrow flesh into chunks and place in a saucepan. Cover with cold salted water and bring to the boil. Draw the pan off the heat and drain the marrow flesh. This initial blanching helps to preserve any natural green colour in the marrow flesh.
Cream the remaining butter with the flour to make a beurre manié. Add in pieces to the hot soup and stir until the small lumps of mixture have melted into the soup. Bring to the boil, stirring all the time. Simmer for 1 minute, then draw the pan off the heat. Cool, stirring occasionally to prevent a skin forming, then chill until ready to serve.