In a large pot over medium heat, melt butter. Add flour and stir continuously, cooking for about 1 minute. Start adding the milk a bit at a time, stirring well until smooth before adding more. Once all the milk is added, pour in the chicken stock. Add the diced potatoes, carrots and thyme. Bring soup to a boil, then reduce heat and simmer until potatoes and carrots are tender, about 15-20 minutes, depending on the size of your dice.