Preheat the oven to 350 degrees F.
Heat 1 tablespoon oil in a large cast-iron skillet over medium-high heat. Sprinkle the steaks with salt and pepper and cook, flipping as needed, until browned all over and cooked to medium-rare (an internal temperature of 130 to 135 degrees F), 7 to 8 minutes. Transfer the steaks to paper towels to drain, wipe the skillet clean and return to medium heat.
Heat the remaining 3 tablespoons oil in the skillet, then add the mushrooms. Sprinkle with salt and pepper and cook, stirring, until golden brown, 4 to 5 minutes. Add the onion and peppers, sprinkle with salt and pepper and cook, stirring, until lightly charred and soft, 7 to 8 minutes. Transfer to a bowl.
Wipe out the skillet and sprinkle the cheese in it. Warm in the oven until melted, about 3 minutes.
Meanwhile, divide the onions and mushrooms among the rolls. Very thinly slice the steaks and them divide among the rolls. Evenly pour some of the cheese over each roll. Sprinkle with flaky sea salt.
Raclette cheese -- which is also the name of a Swiss dish of melted cheese -- is a semi-hard cow's milk cheese made on both the Swiss and French side of the Alps. It has a creamy texture and a nutty flavor.
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