As the train pulled into Peradeniya Junction station in central Sri Lanka, the man sitting opposite me leapt out of his seat and leaned out of the window, placing his thumb and forefinger in his mouth and whistling loudly. A vade seller soon appeared outside, removed a basket from the top of his head and handed it to the passenger.

The man quickly pulled out a fragrant fritter along with a small bag of fiery sambol, leaving money behind, and then passed the basket to other hungry passengers, who did the same before returning the basket back to the seller through the window.

As the train chugged away, everyone settled back in to their seats and contentedly crunched on what I'd later learn were isso vade: lentil patties topped with fresh prawns and deep-fried to create one of the most delicious street foods you could ever find on an island.

Isso (prawn) vade (pattie) are beloved throughout Sri Lanka, and their popularity can perhaps be attributed to their deeply familiar and simple ingredients: lentils and prawns, along with onions and curry leaves. Topped with a spicy sambol – made of chopped onions, tomatoes, green chillies and lime juice – plus chilli sauce for extra punch, each fritter has the perfect balance of crispy texture, zesty aroma and spicy flavour. And at Rs 50 to 70 (12p to 18p) each, they are an inexpensive, tasty treat for the masses.

The most famous isso vade are sold from carts along Galle Face, a seafront promenade in Colombo. Each evening, when the gentle breeze, which has travelled for miles over the Indian Ocean, finally encounters land and cools the city, thousands gather here to spend time with family and friends. They walk up and down the promenade, sizing up each isso vade seller to decide which one has the best offering – usually the one with the largest crowd.