Tomato Rice: In a medium saucepan, heat oil over medium-high heat and add onions and salt. Sauté onions until soft. Add tomato, rice, and broth and bring to a boil. Reduce heat to low and simmer, covered, until all broth is absorbed and rice is cooked. Turn off heat and let sit, covered, 15 minute.
Chimichurri Sauce: Blend cilantro, parsley, oil, lime juice, garlic, agave, cumin, salt and pepper in a food processor until combined and herbs are very finely chopped.
Taco Filling: In a large skillet, heat oil over medium-high heat and saute mushrooms until they have released their juices. Add black beans and half of the Chimichurri sauce and cook until heated through. Reserve the remaining half of the Chimichurri sauce to spoon onto the taco, if desired.
To serve: Fill each tortilla with tomato rice, taco filling, extra chimichurri sauce, desired additional toppings, and a dollop of sour cream. Fold over and serve.