To prepare leeks, cut the root end and about 1/2 cm of the white base. Remove any ragged, coarse outer leaves and discard. Cut the darkest green part of the leaves. The dark green trimmed leaves can be reserved for other uses such as soups. Wash real well and make sure there is no dirt left between the leaves.
First cut vertically all the way to the end. Then chop all the leeks to thin slices.
Add 150 grams of olive oil to a pan and add the chopped leeks.
Add some salt but consider that you will be adding cheese which also has salt. Mix and cover the pan with a lid and cook until the leeks are soft over medium heat. Stir once in a while. The leek will leave off some water very soon, so you do not need to add water.
First wash to get rid of the excess salt and then grate the cheese and add to the leek after the heat is turned off.
Add red pepper flakes and mix well. The filling is ready.