Heat oven to 400 degrees.  Roll pastry into a rectangle on a lightly floured surface.  Place on an ungreased cookie sheet.  Heat cream cheese, milk, and mustard in a 2-quart saucepan over medium heat, stirring constantly until smooth.  Stir in ham and peas.  Spoon the ham mixture down the center of the rectangle.  Lift pastry over the filling. Beat the egg yolk and brush over the braid.  Sprinkle with poppy seeds.  Cut small incisions in to the dough as air vents.  Bake about 25 minutes or until crust is golden brown.