I made an asparagus quiche the other day with 2 or 3 shallots. No reason you couldn't use a leek instead of the shallots.

Just make sure to wash them thoroughly. Slice the leek in half lengthwise. Fan out or completely separate each leaf, and make sure to get all the dirt out from between the leaves. Then I sort of nest them back together, and slice very thin and proceed with my recipe.

Leeks are milder than regular onions.