Put the chicken pieces in a saucepan and cover with milk, then add the onion, peppercorns and bay leaf. Bring to a boil, then lower the heat and simmer partially covered for 20-25 minutes, until the chicken is cooked through. Remove the chicken and let it cool a bit, then pull apart into plump pieces. Strain the milk and save it.
Preheat the oven to 400F. Melt the butter over low heat in a saucepan while you cut the bacon into small pieces, adding it to the pan as you go. Slice the leeks into roughly 1/3" pieces, washing the pieces well after cutting them. Drain, then add the leeks to the pan. Cook the bacon and leeks about 15 minutes, stirring periodically. They should be totally soft.
Stir the flour into the bacon and leeks and cook a few minutes. Then stir in the warm milk, enough to make a thick sauce. Add the chicken pieces, then the mustard and salt and pepper to taste.
Pour the filling into a pie pan or square dish. Lay the puff pastry on top and crimp the edges. Brush the pastry with the beaten egg and then sprinkle grated Parmesan on top.
Bake about 35 minutes.