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Season roast beef with salt and pepper. Heat a large, cast-iron pan and caramelize beef to get some color. Add pre-caramelized onions to the pan to heat through.
Toast the rolls with butter, if desired, and slice in half.
Place 4 ounces of meat on the bread and top with a thin layer of onions, mushrooms (optional) and Gruyre.
Place sandwiches under broiler in the oven to melt cheese.
Mix mustard, mayonnaise and vinegar with chives and spread on sandwiches.