Step 1
Prepare grill for cooking.
Make corn salsa:
Step 2
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.
Step 3
Cook white part of scallions in butter with garlic, 1 teaspoon salt, 1/2 teaspoon each cumin and chili powder, and 1/4 teaspoon 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.
Grill steak:
Step 4
Combine remaining 1/2 teaspoon salt, 1 teaspoon cumin, 1/2 teaspoon chili powder, and 1/4 teaspoon pepper and sprinkle on both sides of steak. Grill, turning once, until an instant-read thermometer inserted horizontally into thickest part of meat registers 130°F, 18 to 20 minutes total for medium-rare. Transfer steak to a grooved cutting board and let stand 5 to 10 minutes before slicing.
Step 5
While steak is standing, reheat corn mixture over moderate heat, stirring occasionally. Stir in cilantro and scallion greens.
Step 6
Spoon corn on top of sliced steak and pour over any accumulated juices.
