Clean the leek: Cut off the white end of the leek with the roots, and slice off the faded green part on the other side. Cut the leeks lengthwise, about halfway down. Take the leaves apart and wash them thoroughly under running water.
Cut the cleaned leeks into roughly ¼-½inch sized pieces. Peel the potatoes and cut them into smaller, about ½inch cubes.
In a saucepan, melt the butter over medium heat. Add the chopped leeks, add a little salt and stir until the leeks are tender. It takes 3-5 minutes.
Sprinkle in the flour and fry for another minute while stirring to form the base for the roux, which will thicken the leek soup.
Add a scoop or two of water and stir to dissolve any lumps.
Now, pour in the rest of the water, carefully put in the chopped potatoes. Salt, bring to a boil.
Turn the heat down to a minimum, cover the pot with a lid. Let it boil gently for about 15 minutes or until the potatoes are soft. Stir occasionally; the flour tends to sink to the bottom of the pot, where it could start to scorch.
In the end, season the soup with crushed or pressed garlic and dried marjoram, which you have first rubbed between your fingers.
Add more salt if necessary. Stir well, and the soup is complete!