Cut the steak along the grain into 3-inch pieces, then slice each piece across the grain into 1/3-inch-thick strips. Toss the steak in a medium bowl with the chili powder, oregano, sugar, 3/4 teaspoon salt and a few grinds of pepper.
Heat a large nonstick skillet over high heat. Meanwhile, wrap the tortillas in a clean dishtowel, and microwave at 100 percent for 1 minute. Let sit, wrapped in the dishtowel, until ready to serve.
When the skillet is hot, add the oil. Add the seasoned steak in an even layer. Don't stir for 2 minutes, then give the steak a good stir and let cook until browned, about 1 1/2 minutes more. Transfer the steak to a medium bowl. Immediately add the onions to the skillet, and stir until brown and slightly softened, about 1 1/2 minutes; transfer to the bowl with the steak, and stir to combine.
Set up a taco bar with the steak and onions, wrapped tortillas, sour cream, guacamole, salsa verde, pico de gallo and cheese.
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