Once it was a treat fishmongers threw in free with the Dover sole, before it vanished from the slabs and re-emerged barely a decade ago as a pick-your-own crop for foodies scouring shorelines and riverbanks. Then, professional foragers started feeding it into restaurants and suddenly samphire became a fixture on the trendiest menus.

Now this delicious British summer vegetable has been introduced to the mainstream - you will find it in many branches of M&S and every Waitrose with a fish counter. It may look odd - green and kind of twiggy - but it has a high vitamin C content, a slight crunch and is a perfect partner for fish and lamb.

Samphire, which was on the menu at Charles and Diana's wedding and even given a name-check by Shakespeare, is a native marsh grass rather than the seaweed it resembles. In the wild - there are beds in East Anglia, Kent, Merseyside and Cornwall - it has all too short a season, appearing in June and gone by September.

However, Waitrose has found a way to keep samphire on the shelves into late autumn by growing their supply under glass in the Midlands: "We simply could not keep up with demand buying from wild sources last year," explains Rhonwen Cunningham, the buyer charged with getting at least 7,000 packs a week to Waitrose's more adventurous customers.

"The fish-buying team were begging us to pursue a consistent source, and when we saw it being featured on TV cookery shows and turning up in so many restaurants and pubs, we realised we were on to a big trend and needed to find a dedicated supply."

Waitrose expects to sell four times as much as it did last year, and has boosted the range with another forager's special, sea aster. Silkier and more leaf-like, it has a creamy as well as a salty quality, which also works as well with lamb as with fish.

Both vegetables are simplicity itself to prepare - just steam for a few minutes and serve. "Samphire does not yield easily to inventiveness," warns Jeremy Lee, of London's acclaimed Blueprint Cafe, who believes it is at its best lightly steamed and buttered "with a pinch of pepper to add a little mystery". In spite of this warning, he likes to combine the salty stems with potatoes and peas in a comforting broth.

At Terence Conran's Shoreditch restaurant, The Boundary, samphire is tossed with lightly sautéed cucumber and served as a side dish, while at Scotts, head chef Dave McCarthy makes a risotto of samphire and girolle mushrooms.

Because it has a good strong flavour, samphire can even take some pickling. Brett Graham of The Ledbury, London's newest entry to San Pellegrino's list of the world's top 50 restaurants, dunks his briefly in two parts apple juice to one part of cider vinegar. Television chef Richard Corrigan combines samphire with spring onions and pickles it in vinegar, sugar and mustard seeds as a robust accompaniment to lamb.

Maria Elia of Joe's, Brompton Cross, who also likes to serve samphire with lamb, suggests blanching it for 30 seconds before tossing with olive oil, garlic, lemon juice, mint and parsley. She also likes to fold it into a quick supper dish of linguine and tinned sardines seasoned with garlic, red chilli and fennel seeds, topped with a big handful of chopped flat-leaf parsley.

This is a good year to introduce yourself to samphire and sea aster, since it cannot be long before many of the less familiar sea vegetables beloved of chefs find their way into the stores. The most popular include sea purslane, which thrives in salt marshes, sea kale, grown on shingle and prized for its beautiful tender purple leaves, and sea beet - a beach plant belonging to the chard family with rich-tasting leaves.
Those who cannot wait can get all these now by mail order from www.forager.co.uk, which also offers instruction to would-be gatherers.