I used a heavy, deep-sided 12-inch skillet for this, but a similarly broad saucepan will be fine.
Set the skillet on a medium heat and add the water. As soon as the water starts to simmer, drop the heat to low-medium and thoroughly stir in the chopped chicken and 1 tablespoon oriental fish sauce. (The other 2 tablespoons of fish sauce are going to be added a little later.)
Keep stirring gently as the chicken cooks on that low-medium heat for 2 minutes.
Now stir in the chilies, red onion, and cilantro. Keep stirring the mix gently for another 2 minutes on low-medium.
Turn off the heat and add 2 tablespoons fish sauce, the lime juice, and the ground, toasted rice. Give the mixture a good stir so that everything is well combined – that’s it, the chicken is done.
You can now let the chicken cool to about room temperature while you prep the lettuce and cucumber, so they’re ready for serving with your larb isaan.