Slice off leek tops and root ends. This recipe needs only the tender, light green part of the leek. Slice leeks lengthwise down the center, clean well under cold, running water and pat dry with a paper towel. Slice leeks horizontally into thin slices and set aside.
In a medium saucepan over medium-high heat, sauté bacon until lightly golden. Add sliced leeks and sauté 3-5 minutes more or until leeks are soft.
Pour in stock, bring soup to a boil. Then lower heat and leave to simmer.
In a separate pot, melt butter over medium heat. Add flour and whisk well until combined. Add to soup, whisking well to avoid lumps. Bring to a boil and season to taste.
Remove from heat. Lightly beat egg yolk and cream together; stir into soup. Serve immediately.