1. Peel and cook the potatoes until tender in salted water. Drain thoroughly, and mash with half of the butter, egg yolks, grated cheese, parsley, salt and white pepper to taste. Set aside.
2. Heat the remaining butter in a frying pan and sauté the onion until tender. Cool slightly and stir in the shrimps, mixing well.
3. Form the potato mixture into balls about the size of a small egg; fill each with about 1 tablespoon of the shrimp mixture. Roll lightly in flour, dip in beaten egg and then in breadcrumbs. Place on a piece of waxed paper and refrigerate for at least 20 minutes. Fry balls in hot oil until golden brown. Drain on paper towels and keep warm. Serve garnished with lemon slices.