To get perfectly thin steak slices, freeze the steak for about an hour before slicing. This little trick makes it easier to slice super thin. Season the beef with salt and pepper, ensuring each slice is evenly coated for maximum taste.

Heat a dry cast iron skillet over medium heat. Add the mushrooms to the pan with a pinch of salt. No need for oil here; the mushrooms will release their own moisture. Cook them for about 5 minutes, stirring occasionally, until golden brown. Once cooked, remove them from the skillet and set aside.

Now, add two tablespoons of olive oil to the skillet. Toss in the onions and sliced bell peppers with a sprinkle of salt. Cook for 3-5 minutes over medium-high heat until the edges begin to brown slightly, but keep their crunch intact. Like the mushrooms, transfer these to a plate once done.

Using the same skillet, heat the last tablespoon of olive oil over high heat. Add the sliced steak in small batches for even cooking. Sear the meat for about 1-2 minutes on each side until it’s browned and slightly crispy. Once all the steak is cooked, set it aside.

With the skillet set to medium heat, toss in the minced garlic. Stir it gently to release its aroma, making sure it doesn’t burn. After about a minute, add the cooked mushrooms, onions, bell peppers, and steak back into the skillet. Stir everything together to evenly distribute the flavors.

Spread the mozzarella and Parmesan cheese evenly over the top of the mixture. Place the skillet under a broiler for 2-3 minutes, just enough to get the cheese melted and bubbly. Keep an eye on it to prevent over-browning.

Sprinkle green onions on top for added color and flavor if desired. Serve warm, either by scooping it into toasted hoagie rolls or as a skillet meal straight from the pan.