MELT BUTTER in a large pot over medium heat. Add the onion and cook, stirring occasionally, until onion softens, about 10 minutes. Add the broccoli, salt and optional Pernod. Continue to cook, stirring, another 15 minutes. Add the stock. Cover, increase heat to high and bring to a boil. Reduce heat to low and simmer 15 minutes. Remove from heat and place in a food processor or blender and puree until smooth. Combine puree, cream and milk. Serve piping hot and offer the grated cheese on the side to add to the soup.