Toast the rolls, split side-up, under a broiler.
Transfer the rolls to a work surface. Using a knife, slather the bottom half of each roll with some of the mustard and top with slices of the beef, overlapping the meat slightly. Season with salt and pepper to taste. 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 1/6 of the roasted shallots and the watercress. Slather the roll tops with the remaining mustard and cover the sandwiches. Slice the sandwiches in half crosswise.
Preheat the oven to 400 degrees F. In a large bowl, toss the shallots with oil, and the salt and season with pepper to taste. Transfer to an aluminum foil-lined baking sheet and arrange in a single layer. Bake until the shallot skins are somewhat charred and crispy and the shallots are very tender, about 40 minutes. Remove from the oven and allow to cool slightly.
Using a knife, slice off the tips of the shallots (the end opposite the root end) and discard. Gently squeeze the shallots from their skins and season with salt and pepper to taste. Serve warm or at room temperature.
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