Preheat the broiler. Spread the rolls out on a pan and toast on both sides.
Slather the bottom 1/2 of each roll with some of the mustard and top with slices of the steak, overlapping the slices slightly. Season, to taste, with salt and pepper. Cover each portion of meat with 2 slices of cheese. Broil until the cheese is lightly browned and melted, about 1 minute.
Transfer the open-faced sandwiches to the work surface. Top each sandwich with some of the roasted shallots and watercress. Slather the tops of the rolls with the remaining mustard and cover the sandwiches. Slice in 1/2 and serve.
Preheat the oven to 400 degrees F.
In a large bowl, toss the shallots with olive oil, 1 tablespoon salt, and pepper, to taste. Spread out on an aluminum foil-lined baking sheet. Bake until the shallots are very tender, about 40 minutes. Set aside to cool.
Slice off the tips of the shallots and discard. Gently squeeze the shallots from their skins and season with additional salt and pepper, to taste.
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