Make chimichurri: Blend cilantro, garlic, oil and lemon juice and salt in blender until smooth. Transfer to a bowl; set aside.
Make steak: Prepare grill or preheat broiler or grill pan. Season steak with cumin, chili powder, salt and pepper. Cook steak over medium flame, uncovered, for 4 minutes. Turn and cook for 4 more minutes. Transfer to board and let rest 5 minutes.
Prepare corn chili: Melt butter in a medium skillet over medium heat, add white of scallion and cook, stirring, for 1 minute. Add corn, red pepper, chili powder and salt; cook, stirring, until corn is crisp-tender, 2 to 3 minutes.
Slice steak; arrange on plates. Spoon chili onto plates; sprinkle with green of scallion. Serve with chimichurri.