Grab your watercress and cut off the root end if you’re using the living stuff. Rinse the leaves well in cold water and spin dry in a salad spinner. (Or just drain the washed leaves in a colander.)
Slice the boneless, skinless chicken thighs into strips, and put them in a bowl. Add the coconut aminos, sesame oil, and fish sauce, and stir to make sure the marinade is well distributed. (If you’re a rockstar meal planner, you can marinate the chicken for up to 24 hours in advance. But if you’re like me, you can do this right before you make the soup.)
Heat the avocado oil in a large saucepan over medium heat and chop up the rest of the vegetables.
When the pan is hot, toss in the shallots, carrots, and shiitake mushrooms and a pinch of salt. Sauté until the shallots and mushrooms are softened (about 5-10 minutes).
Pour in the broth, and bring to a boil over high heat.
Add the chicken and bring everything back up to a boil. Reduce the heat to low (or just enough to maintain a simmer) and cook for 5-10 additional minutes, or until the carrots have softened and the chicken is cooked through. Boneless, skinless chicken breast will cook faster than thighs, so you may need to fish out the chicken first if the carrots aren’t yet as tender as you want ’em. Thighs, on the other hand, are more forgiving if you overcook them, so you can leave them in the pot until the carrots are cooked through.
Remove the pot from the heat and stir in the watercress. As soon as the watercress is wilted, season to taste with more salt and/or fish sauce.
Ladle it up and dig in!