Clean, wash and cut the leek sticks into pieces of about 5 cm length. Cut the remaining leek into fine strips. Cook the leek pieces in bubbling salted water for about 10 minutes until al dente, blanch the leek green for about 1 minute.
In the meantime, dice the ham. Chop the walnuts, except for something to garnish. Mix cream cheese with egg. Fold in ham and walnuts. Rinse the leek pieces and greens with cold water and drain.
Stir the leek green into the cheese mixture. Season with salt, pepper and paprika. Grease the casserole dish. Place the leek pieces in the dish and spread with the cheese mixture. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for approx. 25 minutes.
Sprinkle the remaining walnuts over the gratin 5 minutes before the end of the cooking time. Wash the chives, dab dry and cut into small rolls. Sprinkle chives rolls over the gratin.