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Pat steaks dry with paper towels and season with salt and pepper.
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In a large skillet, heat oil over high. Cook steaks until medium-rare, about 4 minutes per side. Transfer to a cutting board and tent loosely with foil.
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Reduce heat to medium, add shallots, and cook until softened, 8 minutes.
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Add cream and 2 tablespoons water and cook, stirring, until slightly thickened, 1 minute. Stir in mustard and season with salt and pepper. Thinly slice steak against the grain. Make sandwiches with rolls, shallots, steak, and arugula.