In a bowl combine the egg yolk, water, and salt. Stir in flour to make a stiff dough. On floured surface knead until smooth. Divide dough in half. Roll half the dough to less than 1/8 inch thick. Cut into twenty-eight 3-inch circles.
For filling, combine cottage cheese, egg white, sugar, and salt. To make pierogi, place 1 teaspoon cottage cheese filling on half of each circle. Fold dough over the filling to form a half-circle; seal edge with the tines of a fork. Set aside. Repeat with remaining dough and filling.
Cook onion in margarine or butter over low heat about 20 minutes or until very tender. Keep warm.
Meanwhile, in a large kettle bring 12 cups salted water to boiling. Add 10 to 12 pierogis; cook 8 to 10 minutes. Drain on paper towels.
Transfer to a serving dish; keep warm. Repeat until all are cooked. Serve pierogis topped with cooked onions. Makes 7 servings.