Season the steak with the smoked paprika, 1 teaspoon salt, and ½ teaspoon pepper. Heat 1 tablespoon of the oil in a large cast-iron skillet over high heat. Cook the steak, turning once, until medium rare, 5 to 6 minutes. Let rest for 5 minutes before slicing against the grain.
Meanwhile, combine the onion and vinegar in a bowl and let stand for 5 minutes. Stir in the cucumber, tomatoes, ¼ teaspoon each salt and pepper, and the remaining 2 tablespoons of olive oil.
Top the tortillas with the steak, salsa, and Feta.
More like a pita than a taco, but tasty regardless.
