1. In a small bowl, whisk together oil, chile powder, brown sugar, soy sauce, and lime juice. Place steak in a 9-by-13-inch baking dish and cover with marinade. Turn once to coat steak, then cover dish tightly in plastic wrap and refrigerate 1 hour.

2. Heat a grill or grill pan to medium-high. Clean and lightly oil hot grill. Remove steak from marinade, letting excess drip off; reserve marinade. Season steak with salt and pepper. Grill onion and steak, flipping occasionally, until onion is lightly charred and steak is medium-rare, about 10 minutes. Brush onion and steak with reserved marinade and cook 1 minute more, flipping once. Place onion on a serving plate. Transfer steak to a cutting board, loosely tent with foil, and let rest 10 minutes before thinly slicing against the grain. Serve steak and onion with tortillas, salsa, and lime wedges.