Coat both sides of the flank steak with 1 tablespoon each salt and pepper, pressing into the steak. Place in a large freezer bag. Cut both limes in half and squeeze juice into the bag; add chili powder. Mix well, remove air from the bag, and marinate in the refrigerator for about 3 hours.

Preheat an outdoor grill to medium-high heat and lightly oil the grate.

Season bell pepper and red onion with a pinch of salt and pepper. Place them in a grill pan and set aside.

Put black beans in a small pot on the stove over medium heat to warm through.

Cook steak on the preheated grill until browned on one side, 5 to 7 minutes. Spray vegetables with cooking spray and grill alongside the steak, turning often to avoid burning. Cook until both vegetables caramelize and steak is hot, slightly pink in the center, and an instant-read thermometer reads 140°F (60°C) for medium doneness. This should take about 5 to 7 minutes.