In large pot cover potatoes with cold water by 2 inches. Bring to a boil. Reduce heat. Cover and simmer, about 20 minutes or until potatoes are tender when pierced with fork. Drain well in a colander.
Return potatoes to pot. Add milk and margarine. Using a hand masher or potato ricer, mash potatoes until smooth.
Add green onion, garlic, egg yolks, pepper and nutmeg. Mix well.
Gently fold in farmer sausage, cheeses and parsley.
Spoon mixture into greased 2-quart (2 L) baking dish.