Heat olive oil and butter in a large soup pot. Place chicken in the pot and allow to sear for a minute or so on each side. Add a little water if necessary to prevent the chicken from burning. Add chopped onion and cook for a minute or two, moving the chicken around, again adding a little water to prevent burning.
Add the rest of the chopped carrot and celery, and then add enough water to cover the chicken (about 10 cups). Bring to a boil, skim any foam that rises to the top, and then reduce heat to lowest setting. Simmer for 50 minutes to 1 hour, until the chicken is cooked through. Add the collards and parsley to the soup about 5 minutes before it has finished cooking.
Turn off heat. Carefully remove the chicken and allow to cool in a separate bowl.
If you are going to be serving all of the soup right away, you’ll want to remove the meat from the chicken bones and add it back into the soup (make sure you don’t burn yourself).
If you are eating just some of the soup right away, I suggest taking the meat off the chicken and then storing it in the refrigerator to add back into individual servings of soup. You can also reserve some of the chicken for another use.
Just before serving, mix 2 or more tablespoons of coconut milk into each bowl of soup. Add sriracha or garlic chili sauce to taste and sprinkle with minced green onions, if desired.