Melt butter in a Dutch oven over low heat, and whisk in all-purpose flour until smooth. Cook, whisking constantly, 1 minute. Whisk in chopped onion and pressed garlic, and cook 1 minute. Gradually whisk in chicken broth until blended. Add cubed potato and next 3 ingredients. Bring mixture to a boil; cover, reduce heat, and simmer, stirring often, 15 minutes or until cubed potato is tender. Stir in milk, and cook until soup is thoroughly heated.