Heat the olive oil in a large pot over medium heat. Add the diced onion, garlic, carrot, and celery. Sauté for about 5 minutes, or until the vegetables are softened.
Stir in the dried oregano, salt, and black pepper, mixing well to combine with the vegetables.
Pour in the vegetable broth and bring the mixture to a boil.
Once boiling, add the uncooked brown rice. Reduce the heat to low and cover the pot, allowing the rice to simmer for about 20 minutes, or until tender.
After the rice is cooked, stir in the black beans and fresh pesto. Heat through for an additional 5 minutes, ensuring everything is well combined and warmed.
Taste and adjust seasoning if needed. Serve hot, and garnish as desired with grated Parmesan cheese, fresh basil leaves, or croutons.
For a spicier kick, consider adding red pepper flakes while sautéing the vegetables.
You can substitute the brown rice with white rice or quinoa, adjusting the cooking time accordingly.
This soup can be stored in the refrigerator for up to 3 days or frozen for later use. Reheat gently on the stovetop when serving.