Wattana Panich is one of the most popular restaurants in Bangkok’s Ekkamai neighborhood, with hundreds of hungry patrons coming in to feast on its selection of delicious soups and stews every day. But the secret of the flavorsome dishes served at this Thai eatery may put a lot of Westerners off.

One of the most popular dishes at Wattana Panich is the rich beef noodle soup, made with stewed and raw beef, tripe, meatballs, internal organs and spices. But the most important ingredient is the broth, which, believe it or not, has been simmering for 45 years. It sounds strange, but it’s true. Instead of throwing away the leftover broth every night, the owners of Wattana Panich carefully strain it and store it to be used as the base for next day’s batch of soup. They’ve been doing this every day for over four decades and credit it as the main secret to their delicious dishes.

Wattana Panich relies on an old cooking method known as “perpetual stew” or “hunter’s stew” which basically involves leaving the stew to simmer constantly while adding new ingredients to it. This ensures that the broth absorbs as much flavor as possible from the ingredients, making the dishes it is used for absolutely delicious. The principle is simple – the longer the broth simmers, the better it becomes – but this Thai eatery has taken it to the extreme.

According to BK Magazine, the cooks at Wattana Panich cool the leftover broth every night and store it in the fridge to prevent spoiling. It is used as the base for the next day’s stew. The cooks add about 25kg of beef to the stew every day, the flavor of which sips into the decades-old broth, constantly enhancing its flavo

Nattapong Kaweenuntawong is the 3rd generation of his family to be running Wattana Panich, and hopes that his three children will become the 4th. Whatever happens, one thing is for sure, they’ll be using the same broth – or at least a bit of it – as the day the restaurant opened in Ekkamai, 45 years ago.

And in case you’re wondering about that brown hardened shell around the large metal stew pot, it’s testimony to how long the broth has been used for. The owners of Wattana Panich have made it tradition not to clean the 45-years-worth of broth spillover. It’s not the most hygienic piece of history, but it’s history nonetheless.