Place the butter in a large stock pot over medium heat.
Add in the chicken and saute for 5-6 minutes or until the chicken is no longer pink on the outside.
Add the onion, celery, and carrots and saute for 5 minutes stirring occasionally.
Put the garlic in the pan and saute 1 minute.
Add the chicken broth to the pan and simmer for 15 minutes stirring occasionally.
In a medium bowl whisk together the flour and heavy cream.
Pour the heavy cream mixture into the pot with the chicken and vegetables and mix well to combine.
Add in the peas, potatoes, and Montreal seasoning. Stir.
Bring the soup to a simmer and cook for 30 minutes, stirring occasionally.
Taste the soup, season with salt and pepper, and serve.