Tuna Turnovers with Easy Puff Pastry

These Tuna "Empanadas", with a quick Puff Pastry Dough, easy to prepare and without any complications. They are really exquisite, both the dough and the filling. These are traditionally made in Argentina for the Easter Holydays and during this season can be found almost in every bakery.
QUICK PUFF PASTRY DOUGH
400 gr of all purpose flour
400 gr of unsalted butter
1 tsp of salt
2 Cds of lemon juice
12 - 16 Tbs of ice cold water (approx. 250 ml)
TUNA FILLING:
1 small onion
1 Tbsp butter (30 gr)
1 heaped tbsp of Flour (30 gr)
250 ml of milk
1 egg yolk
300 gr canned tuna
1 hard-boiled egg
1 tbsp of chopped fresh parsley
Salt, black pepper and nutmeg