Begin by melting the butter in a large saucepan over medium heat. Add the onion, carrot, celery and garlic and cook until they’ve softened slightly—about two minutes. This builds the flavour base for the chowder.
Stir the flour into the softened vegetables to form a paste. Cook it until the flour smells lightly toasted, about five minutes—watch carefully to avoid burning. Then remove from heat and allow it to cool slightly.
In your large soup pot, combine the diced potatoes, shredded chicken, white and yellow corn kernels, and three cups of the chicken stock. Bring this mixture to a gentle simmer.
In the saucepan holding the roux, whisk in the remaining one cup of stock until smooth, then stir this mixture into the soup pot. Add the half-and-half, nutmeg, salt and black pepper. Bring to a boil, then reduce heat and simmer until potatoes are tender—about 20 minutes.