Make the tomato-chile puree: Place the tomato, vegetable stock, pasilla chile, and oregano in a blender. Blend until smooth and set aside.
Make the soup: Place a saucepan or stockpot over medium-high heat and add oil to coat the bottom. Add the carrot and onion and saute until lightly browned.
Add the tomato-chile mixture, salt & pepper. Bring to a simmer and taste for seasoning. Add more if needed.
Simmer slowly for about 10 minutes to develop flavor.
Serve hot and garnish with the tortilla strips, avocado & cheese. Squeeze some lime juice (highly recommended)