Heat a dry large cast-iron skillet over moderately high heat until hot, then pan-roast corn, stirring occasionally, until golden brown, 8 to 10 minutes. Transfer to a bowl.

Cook white part of scallions in butter with garlic, 1 tsp. salt, 1/2 tsp. each cumin and chili powder, and 1/4 tsp. pepper in skillet over moderate heat, stirring, until scallions are tender, 3 to 4 minutes. Remove from heat and stir in corn, tomatoes, and jalapeños.

Combine remaining 1/2 tsp. salt, 1 tsp. cumin, 1/2 tsp. chili powder, and 1/4 tsp. pepper and sprinkle on both side of steak. Grill steak on about 2-1/2 minutes on each side for medium rare. Transfer steak to a cutting board and let stand 10 minutes before slicing.

While steak is standing, reheat corn mixture over moderate heat, stirring occasionally. Stir in cilantro and scallion greens.

Spoon corn on top of sliced steak and pour over any accumulated juices and garnish with lime wedges.