In a large pot, add onions, butter, and olive oil along with the whole chicken. Pour in enough water until the chicken is almost submerged.

Bring the mixture to a boil, then lower the heat to a simmer for 1½ hours until the chicken is cooked through and tender.

Carefully remove the chicken, then strain the cooking liquid into a bowl. Reduce it over medium heat until it becomes gelatinous.

Allow the chicken to cool slightly, then remove the meat from the bones. Chop and mix with the reserved onions.

In the chicken mixture, add eggs and the reduced chicken stock, seasoning with salt and freshly ground black pepper.

Preheat your oven to 230°C (446°F) and grease a deep pie dish with olive oil.

Layer by placing 3 sheets of filo pastry in the dish, brushing each with olive oil. Add the chicken mixture on top.

Cover the filling with another 3 layers of filo, sealing the edges. Brush the top with olive oil.

Bake for 15 minutes, then reduce the temperature to 200°C (392°F) and bake for an additional 45 minutes until golden brown.

Allow to rest for 10-20 minutes before serving warm.