Preheat a grill. Slice the steak into 1/4-inch thick slices. Slice the cheese into 12 pieces. Set the rolls, cut side-up on a baking sheet and toast under the broiler. Set the tops of the rolls aside.
Slather the bottom halves of the rolls with half the mustard and top with the slices of steak, overlapping them slightly. Season with salt and pepper. Cover with 2 slices of cheese. Broil the open-faced sandwiches until the cheese is lightly browned and melted, about 1 minute.
Transfer the sandwiches to the work surface. Top each sandwich with an equal portion of the roasted shallots and about 3/4 cup of the watercress. Slather the tops of the rolls with the remaining mustard and cover the sandwiches. Slice the sandwiches in half crosswise. Serve immediately.
To make the shallots: Preheat the oven to 200°C. In a large bowl, toss the shallots with the oil and salt, and season generously with pepper. Transfer to an aluminum foil-lined baking sheet and spread out in a single layer. Bake until the shallots? skins are somewhat charred and crispy and the flesh is 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 with roasted beef, chicken, or on steak sandwiches.