Wash the chicken and place in a saucepan. Add the onions, peppercorns, celery and 3 cups boiling water. Cover and simmer gently over a low heat for about 2 hours or until chicken is tender. Add salt during the last hour. Remove from heat. Discard skin and bones and dice meat. Return to saucepan, add 2 tablespoons of the butter and cook with the onions until stock has evaporated. Cool and add the eggs, cheese, bechamel sauce and nutmeg.
Butter a 9x13 inch baking pan. Line the bottom and sides of the pan with 5 to 6 pastry sheets, brushing each sheet with butter. Spread chicken filling evenly over the pastry and cover with remaining pastry sheets, brushing each sheet with butter. Cut 3 inch strips though the three top pastry sheets diagonally across pan. Brush top with butter and sprinkle with water Bake in a moderate oven for about 45 minutes. Cut into diamond shaped pieces. Serve hot.