In a large pot over medium heat, warm the oil. Add the onion, garlic, and salt and cook until softened, about 5 minutes, stirring occasionally. Stir in the red pepper flakes, if using, and cook for 30 more seconds.
Add the water, beans, chicken and soy sauce. Bring to a boil over high heat, then reduce to low/medium-low and simmer for 10-15 minutes or until the chicken is cooked through. Transfer the chicken to a cutting board to shred up.
Use an immersion blender to blend the beans/onion into the broth (or transfer about half or ¾ of soup to a blender and blend until smooth, then pour back into the pot). Remove about 1 cup of broth to use for the miso later, then stir in the greens and simmer until greens are wilted, about 3 more minutes. Turn off the heat.
Add miso paste to a medium bowl, transfer the reserved broth into the bowl, and use a fork (or your immersion blender) to mash it up into a smooth paste. Transfer back to the pot along with the shredded chicken, lemon juice, and butter and stir until butter is melted.
Serve topped with lots of herbs and sriracha or chili oil (I used the Chili Onion Crunch from Trader Joe’s), if using.