Beef is an excellent source of vitamin B12, essential for energy.
Preheat grill to high (450°F to 550°F). Sprinkle steak with salt, pepper and cumin. Rub steak with onion and lime zest. Generously oil grill grate with grapeseed oil. Grill steak, covered with grill lid, until done, 3 to 4 minutes per side. Remove from grill; let stand 10 minutes before thinly slicing across the grain.
Stir together tomato, radishes, lime juice and olive oil in a small bowl. Divide steak evenly among tortillas; top with tomato mixture, cilantro and cheese. Serve with lime wedges.