Preheat the air fryer to 400F.

In a medium bowl, toss the 16 oz sirloin steak cubes with 1 tbsp olive oil, 1 tsp kosher salt, and 0.5 tsp black pepper until well coated.

Place the seasoned meat in a single layer in the air fryer basket and cook for 5 to 7 minutes for medium-rare, shaking the basket halfway through the cooking time.

While the meat cooks, whisk together the 3 tbsp melted butter, 1 tbsp minced garlic, and 1 tbsp chopped fresh parsley in a large bowl.

Transfer the cooked beef directly into the garlic butter mixture, toss well to coat completely, and serve immediately.