This is a brightly flavoured dish that balances a lot of spice with cooling fresh mint and salad greens. I like to serve it as a starter, but it's also good as part of a Thai meal, with other dishes and steamed sticky rice.
Fresh shrimp are easy to find in Asian markets, and although they take more time to prepare than frozen - because you have to remove the shells and heads, then devein them - the results are far superior. As with all seafood, overcooking will make the shrimp tough.
The shrimp are fried in Thai tempura flour, which gives a hard crunch that is very different from what you get with Japanese tempura flour. It's often referred to as Gogi flour, from the name of a popular producer of the flour in Thailand.
Remove the heads and shells from the shrimp but leave the tails on. (Use the heads and tails to make shrimp stock for another dish.) Make a shallow, small cut in the back near the tail and pull out the vein. Put the shrimp belly side-up on a cutting board. Without cutting all the way through the flesh, slice the shrimp down the belly so the body opens up and flattens. Make a 1cm (7/16in) slit in the middle of the shrimp, cutting all the way through to the back. Pull the shrimp tail through the slit from the front to the back, to create a firm “knot” (this helps to keep the shrimp flat when it cooks).
Roughly chop the bird’s-eye chillies. Thinly slice the shallots. Cut the spring onions into 5mm (¼in) pieces. Trim the lemongrass stalks so only the juicy bottom 6cm (2⅓in) remains, then cut into very thin rounds. Put the chillies, shallot, spring onion and lemongrass into a bowl and mix well. Roughly chop the fresh coriander but keep it separate.
Crumble the palm sugar into a bowl, add the fish sauce and lime juice and stir until the sugar dissolves. Taste the mixture and adjust the seasonings, if necessary.
Pour cooking oil to the depth of about 4cm (1½in) into a pan and heat to 180°C (350°F) over a medium flame. Put some of the tempura flour onto a small plate. Coat the shrimp lightly but evenly with the flour and shake off the excess. Fry the shrimp in batches for a minute or two, or until cooked; do not crowd the pan. Drain the fried shrimp on paper towels.
Line a serving plate with butter lettuce leaves. Place the shrimp on the lettuce leaves. Add half the chilli/shallot/spring onion/lemongrass mixture to the bowl holding the palm sugar, fish sauce and lime juice. Stir well, then spoon this mixture over the shrimp.
Arrange the cashews, chopped coriander and whole mint leaves over the shrimp. Scatter some (or all) of the remaining chilli/shallot/spring onion/lemongrass over the ingredients and serve.