Step 1: Preheat your air fryer to 400°F (200°C) and allow it to preheat for at least 3 minutes to ensure even cooking and immediate searing.
Step 2: In a large bowl, toss the 1 ½ lbs of 1-inch cubed potatoes with 1 tbsp olive oil, ½ tsp salt, ¼ tsp black pepper, ½ tsp garlic powder, and ½ tsp dried rosemary or thyme (if using). Ensure they are evenly coated.
Step 3: Place the seasoned potatoes in a single layer in the air fryer basket. Cook at 400°F for 10-12 minutes, shaking the basket vigorously halfway through to promote even crisping.
Step 4: While the potatoes are cooking, prepare the steak. In a separate large bowl, combine the 1 ½ lbs of 1-inch cubed sirloin steak with 2 tbsp olive oil, 1 tsp salt, ½ tsp black pepper, 1 tsp garlic powder, and 1 tsp smoked paprika. Toss until each steak cube is thoroughly and evenly coated.
Step 5: After the initial potato cooking time, carefully add the seasoned steak bites to the air fryer basket alongside the partially cooked potatoes. Arrange them in as much of a single layer as possible. Do not overcrowd; cook in two batches if necessary.
Step 6: Continue cooking the steak and potatoes together at 400°F for another 8-10 minutes. Shake the basket halfway through this cooking phase. Cook until the steak reaches your desired doneness (e.g., 135°F-145°F for medium) and the potatoes are tender inside with crispy, golden-brown exteriors.
Step 7: While the steak and potatoes finish cooking, prepare the garlic butter sauce. In a small bowl, combine 3 tbsp melted unsalted butter, 3 minced garlic cloves, and 1 tbsp fresh chopped parsley. Warm the mixture slightly to blend the flavors. Add optional red pepper flakes or lemon zest if desired.
Step 8: Once the steak and potatoes are cooked, immediately transfer them to a large serving bowl. Drizzle generously with the prepared garlic butter sauce. Gently toss to coat every piece. Serve immediately with additional fresh parsley or grated Parmesan if desired.