What spices go with leek
Quick answer
Thyme, Nutmeg, White pepper — in roughly that order of how well established each combination is.
Each is explained below, with the cuisines it belongs to and the point at which it stops working.
Why leek behaves as it does
Leek is already an aromatic, which is the thing to hold on to: it is doing the job that in another dish would be done by a spice, so it needs supporting rather than seasoning. Its sulphur compounds are milder than onion and its sweetness higher, and both are developed by slow cooking in fat rather than by browning. The reliable additions are gentle — thyme, bay, white pepper, nutmeg in a cream sauce — and the failure mode is a spice assertive enough to make the leek pointless, since a leek buried under cumin might as well have been an onion.
Strong
Thyme
StrongThe reliable partner for a vegetable that is already an aromatic. Leek is doing the job a spice would do in another dish, so it needs supporting rather than seasoning, and thyme is resinous and slow enough to sit underneath it through a long soft cook without competing.
How: With leeks braised slowly in butter; In a leek and potato soup
Good
Nutmeg
GoodAgain a dairy pairing rather than a vegetable one. Leeks in a cream sauce or a gratin take a scrape of nutmeg for the same reason spinach and cauliflower do — the warmth reads against the dairy, and the leek's sweetness is what makes it work here where it would not on a sharper allium.
How: In a leek gratin or cream sauce
White pepper
GoodChosen over black for a pale dish, and the choice is audible as well as visual: white pepper's fermented note sits better with the sweetness of slow-cooked leek than black pepper's sharper bite. This is the classic vichyssoise seasoning.
How: In vichyssoise and cream soups
Cuts and forms
Whole, Whites only, Baby leek. These do not all take the same spicing — the notes above flag where the difference matters.