Cheesy Ground Beef Tacos

In a large skillet over medium-high heat, cook onion until soft, 6 minutes. Add beef and cook until no longer pink, 5 to 7 minutes more, then add tomatoes, black beans, and taco seasoning and season with salt. Stir until combined. Add cheese and stir until completely melted. Spoon mixture into flour tortillas and fold. Garnish with green onions and serve with sour cream.