Steak and roasted corn

Ingredients

3 cups fresh corn (about 3 ears)
4 scallions, white and green parts thinly sliced separately
2 tablespoons unsalted butter
2 garlic cloves, minced
1 1/2 teaspoons kosher salt
1 1/2 teaspoons ground cumin
1 teaspoon chili powder
1/2 teaspoon black pepper
2 plum tomatoes, finely diced
1 to 2 fresh jalapeño chiles, finely diced (including seeds)
1 (2-pound) trimmed boneless sirloin steak, about 1 1/2 inches thick
1/4 cup finely chopped fresh cilantro
Accompaniment: lime wedges