Heat olive oil in a large pot over medium heat. Add onion and mushrooms; cook and stir until softened, 3 to 5 minutes. Pour in tomato juice and vegetable broth. Stir in corn, rice, black beans, tomato, black olives, taco seasoning, and cilantro. Simmer soup until flavors combine, 20 to 30 minutes.
Ladle soup into serving bowls. Garnish each bowl with 1 avocado half.
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