Butcher Chicken Leg Quarters into two pieces: legs and thighs. Place the chicken pieces into the stock pot. Or, leave them in whole. (I find it cooks faster when butchered into two).
Prep Lemongrass: place 1 stalk on a cutting board. Pound with a mallet or rolling pin until cracked and splitting apart. Alternately, grip a knife handle with your fist and pound down with the butt of the handle. The bruised lemongrass should be fragrant. Place in stock pot, on top of chicken. Add onion, salt, and water.
Bring to a boil. Lower heat to medium low and simmer until chicken is tender and the broth looks oily and golden yellow in color, about 30 minutes.
Meanwhile, place dried rice noodles in a large bowl. Add boiling water (I use my tea kettle) until noodles are covered with liquid. Let stand for 10 minutes. Drain and set aside. It's okay if there are a few pieces of noodle that are still hard.
When the soup has finished cooking, remove onion remnants and lemongrass stalks with a slotted spoon. Remove chicken to a plate. Shred meat with two forks, making sure to remove all skin and bones.
Add shredded chicken back into the pot. Add soaked, drained rice noodles and give the pot a good stir. Simmer on medium heat until noodles are tender, about 2 minutes. Taste and adjust seasonings, adding more salt if necessary.
Serve immediately, preferably with a generous sprinkling of cilantro and sriracha.