Cut onions in 2-3 millimetres slices and keep them away.
Roll out the pastry and cut a number of 6cm round bases, by using a glass or a dough cutter. Put an onion slice (containing all its rings) on each pastry base. Add salt, abundant black pepper and a sprinkle of oil.
Bake in pre-heated oven at 190°C for approximately 12 minutes or until brown.
Let them cool, then use 2 spoons to make a tuna spread quenelle, put it on each puff pastry, garnish as you wish and serve.