Simmer the onions in a casserole in the butter on low heat for 30 minutes until the onions have a golden caramelized color and taste. Add chicken stock and thyme. Simmer for 15 more minutes. Taste with salt and pepper. Pour the soup into soup portion bowls and put toasted bread into the soup with grated Jarlsberg® on top. Gratinate in a preheated oven for 10 minutes.