Heat the olive oil in a large pot over medium-high heat; cook the onions and garlic in the oil until the onions are translucent, 5 to 7 minutes. Stir in the black beans, tomato sauce, diced tomatoes, cumin, and cayenne pepper; reduce heat to medium-low and simmer 5 minutes. Add 3/4 cup cilantro and simmer another 2 minutes. Stir in the green onions and remove from heat. Garnish with 1/4 cup cilantro.