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1
Toast the rolls, split side-up, under a broiler.
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2
Transfer the rolls to a work surface.
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3
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.
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4
Season with salt and pepper to taste.
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5
Cover each portion of meat with 2 slices of cheese.
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6
Broil until the cheese is lightly browned and melted, about 1 minute.
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7
Transfer the open-faced sandwiches to the work surface.
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8
Top each sandwich with 1/6 of the roasted shallots and the watercress.
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9
Slather the roll tops with the remaining mustard and cover the sandwiches.
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10
Slice the sandwiches in half crosswise.
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11
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.
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12
Transfer to an aluminum foil-lined baking sheet and arrange in a single layer.
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13
Bake until the shallot skins are somewhat charred and crispy and the shallots are very tender, about 40 minutes.
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14
Remove from the oven and allow to cool slightly.
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15
Using a knife, slice off the tips of the shallots (the end opposite the root end) and discard.
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16
Gently squeeze the shallots from their skins and season with salt and pepper to taste.
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17
Serve warm or at room temperature.