Strangers on the seashore: Dulse, sea lettuce and thongweed are as tasty as they are healthy

The best restaurant starter I have ever had was a piece of grilled fish in butter sauce at the Sportsman pub near Seasalter in Kent. Three years on, the sweet, firm flesh of the slip sole remains fresh in my memory, but what lifted and transformed the fish was the accompanying sauce flecked with tasty green specks.

Every element on my plate was local. The under-regarded slip sole is a common catch in the English Channel, a few feet from the pub. The butter is churned by Stephen Harris, chef-patron of the Sportsman. (His technique: "I just take some cream and beat the crap out of it.") Appropriately for a place called Seasalter, he also makes salt by evaporating seawater. The green specks are the most unusual element of Harris's signature dish. "We're lucky to have sea lettuce on our beach. It is one of the best seaweeds, the king as far as I'm concerned."

"After picking, we wash it thoroughly, though it's not necessary to use seawater as some maintain," explained Harris. "Then it's dried at 80C for about six hours. We crumble it almost to powder and mix it into softened butter."

The specks of maritime greenery deliver a curious potency in both taste and smell. "I once served slip sole in seaweed butter to a great wine expert and he was convinced that the butter was flavoured with truffle," said Harris. "It has hints of truffle and vanilla. Another man started crying when the dish arrived. He told me, 'The smell of seaweed took me straight back to my childhood holidays in Norfolk.'" Imparting that essence of the sea found in oysters and sea urchins, seaweed butter can be used with grilled fish such as scallops and skate or in a pasta sauce with clams or cockles.

You'll certainly have seen, probably trodden on the vivid green leaves of sea lettuce. According to Roger Phillips's guide Seashells and Seaweeds, it is "common in the Baltic, North Sea, English Channel and Mediterranean". Yet it is unlikely that you will have eaten it. The word "seaweed" is an off-putting misnomer and the British tend to ignore these marine vegetables that are as tasty as they are healthy.

One exception is the sensible Welsh. On the fish stalls of the indoor markets in Cardiff, Swansea and Carmarthen, you can see basins of dark green laver or laverbread. This comes from another "very common seaweed" that drapes rather unattractively over rocks at low tide. The limp, dark fronds are just one cell thick, which accounts for this "plastic bag" quality. After being rendered edible by simmering for six hours, the result is a gelatinous slurry. The Welsh customarily coat laver in oats (hence the "bread" part of laverbread) and fry it with bacon, eggs and cockles. In my view, the Welsh breakfast is far superior to the English version.

But this is only the first of a range of options for this healthy delicacy. (Packed with vitamins and minerals, it only contains 10 calories per ounce). Traditionally used as a sauce with lamb or mutton – add the juice and zest of 1 orange and 1 tsp lemon juice to every 125g laver – it can be added to fish soup, kedgeree, omelettes, quiche and mash. A particularly successful application is in pancakes and blinis (see recipe). But you are most likely to have tasted it in a dish from the opposite side of the world: sushi. The Japanese, an island people who recognise the value of seaweed, use it to wrap sushi. Laver is the same as nori.

Such a tasty ingredient is not going to be overlooked by chefs at the top end of the food chain. "I've been using seaweed for years," said Alyn Williams of the Westbury in Mayfair. "Laver is great with smoked fish and eel. I sauté it with shallots and butter then blitz it in the food processor for a few seconds with crumbled toast to absorb the sliminess. The flavour is absolutely delicious. We serve it with ravioli of sweet onions and beach vegetables like sea beet."

There is much more in the tidal store cupboard of seaweeds. "Gutweed, thongweed, dulse..." Julia Horton-Powdrill's list of finds, as she led an event for beginners in seashore foraging at Pembrokeshire Fish Week Festival last month, sounded like a witch's shopping list. Gutweed, whose green strands will have been cursed by many who have slipped on seaside rocks, is particularly unfairly named. "A horrible name but a beautiful taste and aroma," says Stephen Harris. Alyn Williams uses gutweed butter as a sauce for cod with sea purslane. "It is what they should use for Chinese fried seaweed rather than cabbage," explained Julia. Thongweed, also known as spaghetti weed, looks like a cat o'nine tails as it billows in the waves. "Completely delicious when mixed with ordinary spaghetti and boiled."

Her favourite seaweed is dulse. Described as "very common" in UK waters, its broad, blade-like fronds are a distinctive dark purple-red. After washing, it can be used fresh or dry (it does not go crisp but remains pliable). "Fantastic in chicken soup or oxtail stew." The spiciest of all seaweeds is pepper dulse, which is classified as a red seaweed though it is often dull green. "It has quite a strong hit of spicy heat," warned Julia, as she passed me a dried chunk. "Creeps up on you a bit. I put it in a coffee grinder and use as a condiment." Dried seaweed of all types can be sprinkled on to salads or in soups.

If you are going to pick seaweed, make sure that it is from a clean beach. Try to cut or pull the seaweed above its anchorage (known as a standfast) so it can grow again. All the types mentioned here can be identified on Google. However, you don't have to get your feet wet in order to acquire seaweed – 120g tins of laverbread are produced by Parsons Pickles of Carmarthenshire and available by mail order (laverbread.com) or from Amazon. Dried seaweed is available from a number of online suppliers.

Seaweed pancakes

Put 125g plain flour in large bowl. Lightly salt and make a well in the middle of the flour. Add one whole egg and one egg yolk into the well.

Slowly add 225ml of milk plus 2tsp of water, whisking as you do. Whisk until smooth then stir in 120g of laver. Leave to cool in the fridge for an hour.

Melt a little butter in a frying pan. Pour off excess so pan has just a light coating. Heat pan and using ladle or spoon, add enough batter to make a small pancake. Flip over with a spatula when lightly browned.

Serve the seaweed pancakes stuffed with bacon bits and lightly warmed cockles. The batter can also be used to make small blinis as an accompaniment for smoked salmon or eel.