Melt butter in a sauté pan over medium-low heat. Add onions and cook until translucent but not browned, about 4-6 minutes, stirring often.
Add to slow cooker along with diced potatoes, 3 cups of milk, chicken stock, salt and pepper. Cover and slow cook on low heat for 4 hours. Remove lid and check potatoes. They should be soft when pierced with a fork.
Mix reserved 1 cup of milk and cornstarch with a whisk until well blended. If you want to add a little more rich creaminess to this soup, you can add a couple of tablespoons of heavy cream to this cornstarch mixture. Add mixture to soup and continue to cook on low heat for additional 10 minutes, or until soup is thickened. * I pour the soup into a do this on the stovetop right before serving.