Preheat oven at 400 °F (205 °C). In a pan, heat butter at medium heat. Cook leeks for 2 to 3 minutes. Remove from heat and cool down.
Add egg, ham and, if desired, basil. Salt and pepper. Stuff cannellonis with the preparation. In a gratin plate of 33 cm x 23 cm (13 inches x 9 inches), pour half the tomato sauce and set cannellonis. Cover with remaining tomato sauce and mozzarella.
Cover with aluminium paper and oven bake for 25 to 30 minutes. At mid-cooking, remove aluminium paper.
Did you know that...
If you cut off the green part of the leek and put the white part (the stem) in water with the roots, it will grow back!