Start by slicing the onions and green onion. This will ensure they cook evenly and blend perfectly into the sandwich.
In a large skillet, heat the extra-virgin olive oil over medium heat. Once hot, add the sliced medium onions and sauté them until they become caramelized and golden brown, about 10-15 minutes. Stir occasionally to prevent burning.
While the onions are cooking, season the sirloin steak on both sides with garlic salt and freshly ground pepper. This will enhance the steak's natural flavor.
Once the onions are caramelized, push them to the side of the skillet and add the seasoned steak. Cook the steak for about 4-5 minutes on each side for medium-rare, or longer if you prefer it more well-done. Allow it to rest for a few minutes before slicing.
Slice the steak against the grain into thin strips. Toast the hoagie buns until they’re golden and crispy. Layer the sliced steak onto each bun, top with caramelized onions, and add 2 slices of provolone cheese over the meat.
Place the assembled sandwiches back in the skillet (or a clean one) over low heat. Cover the skillet with a lid to help the cheese melt. After a few minutes, check to see if the cheese is gooey and melted to your liking.
Once the cheese is melted, remove the sandwiches from the skillet. Top with sliced green onions if desired, serve hot, and enjoy your delicious Grilled Cheese Steak Sandwich!