1Heat avocado oil in a skillet over medium-low heat. Add the diced onion with a little salt and pepper. Cook until caramelized, adding small splashes of water whenever the pan starts to dry out.
2Thinly slice the sirloin and season with salt, pepper, and garlic powder. Heat a little avocado oil in a hot skillet, spread the steak out, and sear for 45–60 seconds. Flip, break it apart, and cook another 1–2 minutes until just cooked through.
3Add the caramelized onions back to the steak and mix together. Top with the provolone, let it melt, then mix the melted cheese into the steak and onions.
4Slice the hoagie rolls about ¾ of the way through, keeping them attached. Mix the softened butter with the garlic and parsley, spread it over the inside of the rolls, then toast until golden.
5Make the cheese whiz last. Add the American cheese, milk, butter, garlic powder, and Worcestershire to a small saucepan over low heat. Stir until completely melted and smooth.
6Fill the toasted hoagie rolls with the cheesy steak and onions, then spoon the warm homemade cheese whiz over the top.