To prep the leeks, cut off the dark green leaves at a 45-degree angle and discard. Chop the white and light green parts, wash well in a bowl of water and spin dry in a salad spinner. Wet leeks will steam rather than sauté. For the puff pastry croutons, defrost puff pastry overnight in the refrigerator. You want the pastry to be very cold when you bake it.