In a small bowl mix salt, cumin and smoked paprika. Coat both sides of flank steak with the spice rub and let sit at room temp while you make the salad or up to 30 minutes.
Make Corn-Radish Salad. Shuck and clean corn, wiping off stray threads with a kitchen towel. Grill it lightly- just for a few minutes on each side. Cut kernels off with a large sharp knife and place into a large bowl. Thinly slice radishes and place into the bowl. Very thinly slice onions and place in the bowl. Add chopped parsley. Add the lime juice, olive oil, salt, pepper and coriander and toss well. Set aside or refrigerate for up to 3 hours.
Grill meat on high heat until steak is nicely charred on each side and cooked to medium rare, about 5 minutes each side. Let stand 10 minutes before slicing.