Its called gaeng pak (or keng phak). Pak means vegetable. If its mild its gaeng jeut (mild). You put water in a pot, simmer it with a little bit of galangal or ginger and lemon grass and a chilli to flavour it lightly. Add a bunch of greens (like false pak choi), taste and add salt and/or fish sauce or if vegetarian a slosh of soy sauce. Thats it. If the greens are small and fresh it tastes delightfully sweet. The simplest version is just water, greens and salt.