On a cutting board, with a sharp knife, slice the green, red and yellow peppers into thin strips and finely chop the spring onions.
Heat a large deep non-stick frying pan over medium heat, add the butter, and sauté the peppers for 3–4 minutes to release their aroma, seasoning with salt and freshly ground pepper.
Transfer the peppers to a large bowl and add the chopped spring onions and the corn.
Bring the chicken stock to a boil in a small saucepan over high heat. Add the chicken fillets and simmer for 12–14 minutes.
Once cooked, remove the fillets from the saucepan, place them on a cutting board, and cut them into small cubes with a sharp knife.
Add the diced chicken to the bowl with the vegetables. Stir in the grated kasseri, the two eggs, salt and freshly ground pepper, mixing thoroughly.
Lightly flour a clean countertop to roll out the first puff pastry sheet and lay it on a non-stick baking tray.
Spread the chicken filling evenly over the pastry sheets and cover with the second sheet.
Fold any overhanging edges inward to form a border, then brush the top with the beaten egg yolk.
Bake in a pre-heated oven at 180 °C for 40–45 minutes until the puff pastry is cooked through and the pie is of an appealing golden color.
