Preheat your skillet on the stove over medium-high heat.
Slice the flank steak into thin strips.
Pour the vegetable oil into the skillet, then add the diced onion and peppers to the pan. Cook for 3 to 4 minutes, stirring occasionally, until the veggies are slightly softened.
Add the meat to the pan; season with salt.
Cook the meat until it is browned all over. This should take about 5 minutes.
Mix the beef strips and peppers in the skillet, and divide the mixture into two equal portions on opposite ends of the skillet.
Place a slice of cheese onto each mixture.
Once the cheese begins to melt, place each half into a split open hoagie roll.