Created by ForeverMama on April 2, 2021
Step 1: Place the noodles in a large bowl and pour boiling water over them to cover. Help them soften by pushing them into the water with a wooden spoon or rubber spatula. Let sit for 4 minutes until soft, then drain. Toss with 1 tablespoon of the vegetable oil to keep the noodles from sticking together, cover with plastic wrap to keep the noodles from drying out, and set aside.
Step 2: Peel and discard the dry, papery outer layers of the lemongrass; trim off the top two-thirds of the stalk, which is also dry and papery, and the very base and discard. Finely chop the bottom third (5 to 6 inches) of the stalk, where the stem is pale and bendable. You should have about 2 tablespoons of chopped lemongrass.
Step 3: In a large stockpot, combine the lemongrass, stock, 10 of the garlic cloves, the sliced ginger, lemon juice, and fish sauce and bring to a boil over high heat. Reduce the heat to medium and simmer for about 30 minutes. You should be able to smell the broth from the next room. Season with 1 tablespoon of the salt and 1 teaspoon of the pepper and taste for seasoning. Does it need perking up? Add a pinch more salt. You know what you like and what tastes good. When it is delicious, strain it through a fine-mesh strainer, discard the solids, and place the broth back on the stove over low heat to keep it hot.
Step 4: Using a vegetable peeler or a small sharp knife, peel the chayote. Cut it in half, use a spoon to dig out the pit and cut each half in half. Slice each piece into 1/4-inch half-moons and set aside. (If you don't have chayote, trim the squash, halve them lengthwise, slice the halves into 1/2-inch half-moons, and set aside.)
Step 5: In a wok or a huge, heavy, flat-bottomed skillet, heat the remaining 2 tablespoons vegetable oil over high heat until it simmers, about 1 minutes. Thinly slice the remaining 2 garlic cloves and add with the chopped ginger to the wok. Stir with a wooden spoon until golden, 1 to 2 minutes. Season the chicken breast strips with the remaining 1 teaspoon salt and 1/2 teaspoon pepper and add to the wok. Stir until the chicken turns opaque and starts to caramelize, 3 to 4 minutes. Add the chayote and about 2 cups of the chicken broth and simmer until the vegetables start to soften (you want them to have a little texture) and the chicken finishes cooking, 3 to 4 minutes.
Step 6: Divide the rice vermicelli noodles among four to six deep soup bowls. Divide the chicken among the bowls. Tear up the watercress and add a handful to each bowl, using up all the watercress. Ladle the remaining broth into the bowls evenly. Right before serving, squeeze the lime wedges into the soup.