Heat a large skillet or griddle. Lightly brush a tortilla on both sides with oil and lay it in the skillet. Cook over moderate heat for 45 seconds, then flip the tortilla and sprinkle 1/4 cup of the mozzarella over half of it. Top the cheese with a quarter of the sirloin cubes and avocado slices. Sprinkle with another 1/4 cup of cheese. Fold the tortilla over the filling and press down with a spatula to form a taco. Continue cooking for 30 seconds, then turn and cook until the underside is golden, 1 minute longer. Keep the pirata warm in the oven while you repeat with the remaining tortillas and fillings. Serve warm, with salsa.