place cubed squash in medium saucepan with water and a pinch of salt and bring to a boil over high heat. lower heat, cover, and steam till squash is tender, about 6 minute remove squash with slotted spoon and set aside.
add green beans to pan. repeat process with green beans, topping with water as needed.
return squash to pan with green beans and remaining cooking liquid (there should be just a little). add coconut milk and a bit more salt, if desired. bring to boil and immediately turn heat down to low. simmer curry, uncovered, about 8 min or until slightly thickened--but do not allow mixture to come to a rolling boil or it will curdle. don't stir because squash might discintegrate; shake the pan if you need to mix ingredients.
transfer curry to serving dish.
heat oil in small skillet over high heat. when oil starts to smoke, add mustard seeds and cover pan. after seeds stop sputtering, add chiles and cashews and shake pan over medium heat till cashews are toasted and lightly browned. pour this over curry and serve.