Peel and finely chop the onion and leeks. Press or grate the garlic. Sauté the onion, leeks and garlic in butter over medium heat and, when softened, add the flour, stir, and slowly add milk. Leave the mixture for a few minutes over medium heat until it thickens, stirring continuously to avoid lumping. Empty the sauce into a bowl and add the chicken, Parmesan cheese, breadcrumbs, lightly beaten eggs, herbs, and stir. Following the instructions, make the triangular pies and bake them in a preheated oven at 200°C for 30 minutes.

For making Filo Pastry Triangles:
Cut the sheets lengthwise into 3 strips. Brush 3 sheets lightly with butter. Place 2 tablespoons of the chicken mixture on one corner of the sheet. Cover the filling by folding the sheet in a triangular shape. Continue folding into a triangle. Repeat with remaining Fillo pastry sheets and filling.