Poach the chicken breasts: place the chicken breasts in a pot large enough so they’re sitting on the bottom of the pot; pour in just enough water to cover the chicken pieces; add the piece of lemongrass, lime leaves and knob of ginger; bring the water to a boil, then immediately reduce the heat to low, and simmer until when you poke a thermometer into the widest part of the breast the internal temperature reaches 74°C/ 165°F. Remove and when they’re cool enough to touch, shred the chicken pieces into strips using your hands or two forks.
Make the dressing while the chicken is poaching: to a jar with lid, add two tablespoons of quality fish sauce, the juice of one lime, a teaspoon of palm sugar, half a teaspoon of sea salt and half a teaspoon of ground black pepper, pop the lid on and shake vigorously, then remove the lid and set aside so the flavours meld together.
To a large mixing bowl, add the torn Romaine lettuce leaves, cucumber pieces, sliced spring onions/scallions, finely sliced fresh red chillies, sliced purple shallots, minced garlic clove, and half of each of the portions of fresh coriander leaves, fresh mint leaves and fresh dill sprigs.
Just before serving, add the shredded chicken, evenly pour the dressing over the salad, use a big spoon or clean hands to thoroughly combine, then transfer to a salad bowl or serving dish, sprinkle on the remaining fresh herbs and crunchy fried shallots, and serve immediately.