Start a fire (+/- 180°C or 350°F)
Once the coals are inflamed, put the onion, sweet bell peppers and fennel between the coals. Don't worry, they need to burn on the outside to enhance the taste of this soup.
Once the vegetables are black remove them from fire. Put the bell peppers in a plastic bag and let them cool down. This is verry helpful to remove the skin.
You can start removing the skin from the vegetables once they have cooled down some time! If you would start immediately you risk getting burned.
Take a cast Dutch Oven and put it on the fire. Pour in some oil and add the garlic and thyme.
Cut the vegetables roughly and add them to the Dutch Oven. Let them simmer for a couple of minutes and add the fish sauce.
Add chicken broth to the dutch oven and let the soup cook for +- 10 minutes.
Mix the soup and spice it at taste with Basil and Cayenne Pepper.
Add the Lemon juice and taste if the soup needs something more.