West 52nd Street Steak Sandwich

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6 crusty rolls, such as a Portuguese or country sourdough, split in half lengthwise

1/3 cup Dijon mustard

1 1/2 pounds grilled medium-rare steak, preferably from porterhouse, hanger, or ribeye, thinly sliced

Kosher salt and freshly ground black pepper

3/4 pound Saga blue cheese, cut into 12 slices

ROASTED SHALLOTS, roughly chopped, recipe follows

4 cups loosely packed watercress, washed and dried

Roasted Shallots

18 shallots (about 1 3/4 pounds), unpeeled

1/4 cup extra-virgin olive oil

1 tablespoon kosher salt, plus more

Freshly ground black pepper