Combine chicken broth, chopped leek, celery and onion in a saucepan.
Bring to a light boil, reduce heat and simmer covered for 45 minutes, stirring occasionally.
Strain. Discard veggies.
Return broth to saucepan. Heat over med/low heat.
In a small bowl, combine cornstarch and cold water until smooth.
Stir cornstarch paste into soup. Stir or whisk until soup is smooth and thickened.
Stir in shredded cheese. Stir until completely smooth.
Stir in salt, pepper and nutmeg.
In a separate bowl, combine egg yolk & heavy cream. Whisk with fork.
Add in (approx) 1/2 cup of the hot soup. Stir until well combined.
Add mixture into soup. Stir constantly for about 2 minutes over low heat.
Remove from heat. Stir in 1/4 cup white wine. Serve immediately.
Serve soup with lots of fresh, crusty bread for dipping. Enjoy!