Borough Artisan Sandwich Deli: The £8 Kappacasein Toastie

Kappacasein makes just two hot dishes, and both cost £8. The cheese toastie is mostly Montgomery's cheddar with onions and leek on sourdough; the raclette is melted and scraped over baby new potatoes. This artisan sandwich stall trades at 1 Stoney Street, London SE1 9AA, on Thursday and Friday from 10am to 5pm and Saturday from 9am to 5pm, and it is closed Sunday to Wednesday.

The founder, William Oglethorpe, studied agriculture in the Swiss Jura, according to Kappacasein's own site. In 2008, using surplus cheese and bread from the market around him, he started Kappacasein, selling a toasted cheese sandwich and raclette. The name comes from kappa-casein, one of the proteins in milk, a fair clue that the person behind the stall cares more about cheese chemistry than branding.

Six years after the first toastie, the same history page says, he began making his own cheese. The business now has a dairy site in Bermondsey, and the raclette on the stall's menu is made with London Raclette or Ogleshield, the cheeses named on its own menu. The toastie is therefore a cheesemaker's sandwich rather than a cafe's, and that is the reason people queue for something so plain on paper.

The official menu describes the toastie as a mix of cheeses, mostly Montgomery's cheddar, between two slices of sourdough, with a sprinkling of onions and leek. Borough Market's own trader page lists a slightly wider blend of Montgomery cheddar, Ogleshield, London raclette, Comté, onions and leeks, which suggests the mix shifts with what is in the cheese room that week.

Whatever the exact blend, the method stays the same: the sandwich is pressed on a hot plate until the outside is crisp and the inside runs. The alliums matter more than they sound. Raw onion and leek soften in the heat and cut through the fat of the cheddar, so the toastie tastes sharp rather than just heavy. Nothing else goes in: no ham, no pickle, no mustard on the menu description, which is why the cheese has to carry the whole sandwich.

Order the toastie on a first visit. It is the dish the stall built its name on, and it travels: you can carry it down to the riverside and eat it there. The raclette, also £8, is made with either London Raclette or Ogleshield, melted and scraped over baby new potatoes and finished with black pepper and cornichons. It is excellent, but it needs a fork and a place to put a tray.

If there are two of you, get one of each and swap halfway. The cornichons on the raclette are the only acidity on the menu, and they make a good break from a whole sandwich of cheddar. Skip the idea of a side; there is none on the menu, and two £8 dishes between two people is already a generous lunch. Buy the food first, then wander for coffee once you have it.

The short trading week is the most important thing to know. Kappacasein is closed from Sunday to Wednesday, so a Monday trip to the market will leave you empty-handed. Thursday mornings, soon after the 10am start, are the quietest window, when office workers have not yet arrived for lunch and the weekend crowds are days away.

Saturday is the busiest day, with the stall starting earlier at 9am. If you want a toastie without a long wait, be there in that first hour, before the market fills with weekend visitors. If the line stretches too far, walk the market loop and come back; the stall trades until 5pm. The official site also lists a summer break, closed from the 15th to the 29th of August, so plan around that late-summer gap if you are visiting London in August.

The toastie stall is the public face of a working dairy. Kappacasein's site lists a second site at Spa Terminus, 1 Voyager Industrial Estate, London SE16 4RP, open on Saturdays from 9am to 2pm. That one sells cheese rather than toasties, and it is part of the cluster of food producers under the railway arches in Bermondsey that open to shoppers at the weekend.

If you like the Ogleshield or the raclette on your toastie, the Bermondsey site is the place to buy a piece of it to cook with at home. Combine it with a Saturday morning walk along the arches, then take the train or walk back toward Borough for lunch, and you have tasted the cheese at both ends of its life.

Stoney Street runs through the heart of the market, so you will not have to go far for a view. Southwark Cathedral stands at the market's western edge, a medieval church with a quiet churchyard where people eat lunch on dry days. A few minutes further west along the river, the replica of the Golden Hinde, Francis Drake's ship, sits in its dock in St Mary Overie.

From there, the Thames Path runs west past Shakespeare's Globe and Tate Modern to the Millennium Bridge, a flat riverside walk that turns a toastie into an afternoon. Head east instead and London Bridge station and the Shard are a few minutes away, with Tower Bridge beyond for anyone who wants the full postcard.

Kappacasein, 1 Stoney Street, London SE1 9AA. Open Thursday and Friday 10am to 5pm and Saturday 9am to 5pm, closed Sunday to Wednesday. Toastie £8, raclette £8. No phone number is published, so the stall is walk-up only. The Spa Terminus dairy in Bermondsey opens Saturday 9am to 2pm for cheese sales.

London Bridge, on the Northern and Jubilee lines and National Rail, is the nearest station, a short walk from the stall. Expect to eat standing or perched nearby, as this is a market stall rather than a sit-down cafe. Ask Karpo APP to build a Thursday lunch crawl through the market or to suggest a riverside spot to sit with your toastie.