Heat oil and butter in a large heavy saucepan over medium-high heat until butter melts. Add onion and cook, stirring occasionally, 10 minutes or until tender and almost caramelized. Add celery and cook, stirring often, 10 more minutes. Add garlic and cook, stirring constantly, 30 seconds, being careful not to burn garlic.

Add peppers and next 4 ingredients and cook, stirring, about 2 minutes. Add sweet potatoes, broth and bay leaves; bring to a low boil. Reduce heat and simmer for about 40 minutes or until potatoes are tender. Stir in chicken and cook 2 more minutes. Skim any foam from the top and discard. Remove bay leaves and discard. Divide soup evenly among individual bowls and top with cilantro, lime, avocado and crispy tortilla strips.