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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.
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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.
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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.
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While steak is standing, reheat corn mixture over moderate heat, stirring occasionally. Stir in cilantro and scallion greens.
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Spoon corn on top of sliced steak and pour over any accumulated juices and garnish with lime wedges.