Heat olive oil over medium heat in heavy duty saucepan or Dutch oven. Add mushrooms, carrots and green onions. Cook for 3-5 minutes, or until onions are soft and fragrant.
Add tomatoes, broth, water, cayenne pepper (if using) and Ramen noodles. Bring to a boil and reduce to a simmer over Low heat. Simmer for 15 minutes uncovered. Remove from heat and stir in chicken.
Transfer to serving bowls and top with avocado slices.