Heat a large pot or dutch oven to medium. Add the oil.Add the onion and sauté for 4 minutes, then add the garlic and sauté for a minute more. Add the carrots and continue to sauté for around 5 minutes. It's okay if the vegetables get some color on them, just don't let them burn. Add the harissa and sauté for 2 more minutes. Add the broth and salt and bring the soup to a boil. Once boiling, immediately turn to a simmer and continue to simmer for 30 minutes. Turn the heat to low and use an immersion blender to purée the soup until smooth.