Combine potatoes, carrots, onion, and stock in soup kettle and simmer, partially covered, 15 minutes. As vegetables cook, melt butter in saucepan. Add flour and stir until blended. Slowly add milk, stirring with a whisk, until mixture is smooth. Stir in cheese and cook over low heat until melted. Season with salt and pepper. Slowly add cheese mixture to vegetables and stock, stirring over low heat until blended. Garnish with chopped parsley.