Begin by melting butter in a large soup pot. Add in diced onions, carrots, and celery. Sweat for 5-10 minutes until onions are translucent.
Add in flour and stir for 1-2 minutes. Carefully whisk in milk until thickened.
Add chicken stock, potatoes, shredded chicken breast, salt, pepper, and nutmeg.
Simmer on Medium-Low heat for 20-25 minutes, or until potatoes are cooked through.
Add in frozen peas and corn, and cook an additional 5 minutes.
Serve soup as-is or add butter noodles and cook until noodles are softened.