Cut the sirloin into bite-sized pieces, keeping them roughly the same size so they cook evenly.
In a bowl, whisk together the olive oil, salt, pepper, onion powder, garlic powder, and brown sugar.
Add the steak pieces to the bowl and toss until evenly coated. Let them sit for 15-20 minutes to soak up the flavor.
Preheat the air fryer to 400°F (200°C).
Arrange the marinated steak in a single layer in the basket, leaving space between pieces so air can circulate.
Air fry for 4-6 minutes, checking for your preferred doneness.
While the steak cooks, melt the butter in a microwave-safe bowl, then stir in the minced garlic and parsley.
Transfer the cooked steak to a serving plate and pour the garlic butter sauce over the top, tossing to coat.
Let it rest for a couple of minutes so the flavors settle in, then serve hot.