Use your hands or a knife to remove the chicken meat from the bones and chop or shred it into bite-sized pieces. Try to get as much meat off as possible, but it’s fine if you don’t get every bit. (Note: reserve the chicken bones - you’ll use these in the broth.)
Heat a large pot or Dutch oven over medium heat. Add olive oil and then mirepoix with salt. Saute until vegetables are nearly tender, 5 to 6 minutes.
Stir in miso and garlic, breaking miso up a bit with the back of a spoon or spatula.
Add reserved chicken bones, stock, and bay leaves.
Cover with a lid and cook until the soup begins to boil. Reduce heat under the pot so that the soup stays at a low simmer. Simmer, covered, for 15 to 20 minutes to let the flavors develop.
Remove the lid and discard chicken bones and bay leaves.
Increase heat under the soup so that it is simmering gently.
Stir the chopped chicken and pasta into the soup. (See note below and cook pasta separately if you’re not going to eat all of the soup right away.)
Simmer until pasta is tender (check package for timing).
Stir lemon juice into soup. Taste the soup and, if you want to add more savory flavor / saltiness, stir in up to 2 more Tablespoons of miso.
Stir in chopped parsley (if using) and serve.