What spices go with green beans
Quick answer
Mustard seed, Sichuan pepper, Garlic — 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 green beans behaves as it does
Green beans are close to a blank surface — mild, slightly grassy, with a texture that survives longer cooking than most green vegetables — and that makes them one of the few vegetables that can take assertive spicing without being lost. South Indian poriyal with mustard seed and curry leaf, Sichuan dry-fried beans with chilli and Sichuan pepper, and Turkish beans stewed in olive oil with tomato are three unrelated traditions all treating them as a vehicle. The one consistent requirement is fat: the beans carry aromatics poorly on their own and well once oiled.
Classic
Long-established in at least one cuisine — the combinations you can rely on.
Mustard seed
ClassicThe South Indian poriyal, and it depends on a property most green vegetables do not have: green beans hold their texture long enough to be tempered and stir-fried rather than merely blanched. Mustard seed popped in oil with curry leaf and the beans in after is a complete technique.
How: In a South Indian poriyal with curry leaf and coconut
Strong
Sichuan pepper
StrongSichuan dry-fried beans, and the beans are chosen for the same reason: they survive the hard blistering fry that gives the dish its texture. The numbing works against a very oily preparation in a way an aroma alone would not.
How: In dry-fried green beans with chilli and preserved vegetable
Garlic
StrongNearly universal, and worth recording for the mechanism rather than the novelty: green beans carry aromatics poorly on their own and well once oiled, so garlic in oil is doing the delivery as well as the flavouring. That is why blanched beans tossed in garlic butter work and boiled beans sprinkled with garlic powder do not.
How: Blanched then tossed in garlic and oil
Cuts and forms
French, Runner, Flat, Yardlong. These do not all take the same spicing — the notes above flag where the difference matters.