Place the flank steak in the freezer for 15 minutes to firm up, then slice against the grain into very thin strips.
Peel and slice the yellow onion into thin half-moons.
Heat the extra virgin olive oil in a cast-iron skillet over medium heat.
Add the onions to the skillet and sauté, stirring occasionally, until they are soft and golden brown, approximately 8-10 minutes.
Increase the heat to medium-high and add the steak strips to the skillet with the onions.
Season the mixture with sea salt, black pepper, and garlic powder.
Sear the steak quickly for 2-3 minutes until browned but still tender, then arrange the meat and onions into a pile roughly the size of your roll.
Place the provolone slice over the meat and onion mixture, covering the skillet for 30 seconds to melt the cheese.
Slice the sprouted grain roll and toast it lightly under a broiler or in a separate pan.
Using a wide spatula, scoop the cheesy steak and onion mixture into the toasted roll and serve immediately.