This recipe comes from the World's Most Expensive Cookbook (that's what my family calls it -- you know, receive 36 recipes in the mail every month, pay $10). But this is one of the few that endures year after year as a favorite. . .
Cut potatoes into cubes. Place in a large saucepan. Cover with water. Add a dash of salt.
Bring to a boil. Cover and simmer for about 20 minutes, until potatoes are just tender. Drain.
Mash potatoes. Add green onion and garlic; mix to blend.
Add butter, hot milk, egg yolks, pepper, and nutmeg. Whip until light and fluffy. Add parsley.
Reserve 12 perfect slices of sausage. Heat the rest of the sausage in the microwave for a minute or two (just enough so that it is hot). Add mozzarella cheese, the hot sausage slices, and Parmesan cheese to the mashed potatoes.
Spoon mixture into a buttered ovenproof dish. Sprinkle with herbs. Arrange reserved sausage slices on top like a clock face.
Heat under the broiler until lightly browned and hot, 5 to 7 minutes.