The sweet flavor of slow-cooked leeks complements the woodsy scent of mushrooms especially well in the creamy custard of this quiche. Although it might be delicious to use some fancy wild mushrooms in this, I’ve crafted the recipe with the white cultivated mushrooms available everywhere. I would definitely prepare the leeks and mushrooms the day before to cut down on the last-minute rush. Since they both need to cook slowly for maximum flavor, cooking them separately actually saves you time. This is a perfect appetizer for an elegant dinner.