In a large bowl, combine cottage cheese and eggs. Stir in the flour, salt and 1 tablespoon butter.
In a Dutch oven, bring 3 qts. of water to a boil. Reduce heat. Drop batter by tablespoonfuls into simmering water, stirring gently to prevent sticking; cook pierogi in batches for 4-6 minutes or until slightly firm. Remove with a slotted spoon to paper towels to drain; cool slightly.
In a large skillet, saute onion and pierogi in remaining butter until lightly browned. Serve with sour cream.