Place the flank steak in the freezer for 15 minutes to firm up, then slice it against the grain into very thin strips.
Thinly slice the yellow onion and green bell pepper into long strips.
Heat a cast iron skillet over medium heat with half of the avocado oil.
Add the onions and peppers to the skillet, sautéing for 8-10 minutes until the onions are golden brown and caramelized.
Remove the vegetables from the pan and set aside.
Increase the heat to high and add the remaining avocado oil to the same skillet.
Season the steak strips with sea salt, black pepper, and garlic powder.
Add the steak to the hot skillet in a single layer and sear for 1-2 minutes per side until browned and slightly charred.
Return the onions and peppers to the skillet, tossing them with the steak.
Shape the mixture into a pile roughly the size of your roll and place the provolone cheese slice on top to melt.
Slice the hoagie roll open and toast it lightly in a separate pan or toaster oven.
Use a spatula to scoop the cheesy steak and vegetable mixture into the toasted roll and serve immediately.