Preheat your grill to 400ºF.
In a small bowl combine the onion powder, garlic powder, chili powder, cumin, salt, and pepper. Brush the oil on the flank steak and rub with the spice mixture. Let sit 20 minutes.
Oil grill grates and grill meat until desired doneness, about 5-8 minutes per side. Remove from grill and cover with foil. Let sit 10 minutes. Cut into strips.
Meanwhile grill corn tortillas on the grill until charred.
To serve, place a tortilla on a plate. Top with the meat, jalapeño, and chopped onion. Sprinkle with cilantro.