Heat the oil in a non-stick frying pan. Salt the meat, brown the meat all over for approx. 4 mins. over a medium heat, remove and leave to cool.
Heat the oil in the same pan. Add the leek, sauté for approx. 7 mins., drain if necessary. Stir in the cream cheese, season, leave to cool.
Roll out the dough on a baking tray, cut three strips (each approx. 1 cm wide) from one of the narrow ends and chill. These strips will be used for decoration. Place the bacon in the centre of the dough. Spread 3/4 of the filling on top of the bacon. Place the fillet on top, spread the remainder of the filling all the way around. Brush the edges of the dough with egg white, fold loosely over the fillet, press down firmly. Plait the reserved strips of dough into a braid, brush with egg white and place on the joining seam. Cover the fillet and leave to chill for approx. 1 hr.
Whisk together the egg yolk, water and salt. Brush the dough with the egg wash, prick several times with a fork. Insert the meat thermometer into the thickest part.
Approx. 30 mins. in the lower half of an oven preheated to 220 °C until the core temperature has reached approx. 55°C. Remove the meat and leave to rest for approx. 10 mins. before carving.