Pre-heat the oven to 375.
Place the cubed sweet potatoes in a baking dish with 1 tbsp olive oil and a pinch of salt and white pepper. Toss to coat. Roast the sweet potatoes for 25 minutes, turning once. Remove them from the oven and allow them to cool to room temperature.
While the sweet potatoes are roasting, prepare your flank steak. In a small bowl, combine the salt, garlic powder, cumin and Mexican chili powder. Mix well.
Use your hands to rub the seasoning on both sides of the steak. You can cook your steaks on an actual grill or in a hot grill pan. (Stop panicking, you can also use a regular pan if you have to.)
Cook for approximately 7-8 minutes on each side for medium rare. Remove the steaks from the pan and allow them to rest for ten minutes before slicing.
Add the romaine, red onions, sweet potatoes, cilantro, and scallions to a large bowl.
In a small bowl combine all of the ingredients for the dressing. Pour the dressing over the salad just before serving, and toss. Serve salad topped with sliced flank steak.