For the steak sandwiches:
1) Preheat the grill. Spread the rolls out on a pan and toast on both sides.
2) Slather the bottom half of each roll with some of the mustard and top with slices of the steak, overlapping the meat slightly. Season with salt and pepper to taste. Cover each portion of meat with 2 slices of cheese. Grill until the cheese is lightly browed and melted, about 1 min.
3) 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 half and serve.
For the roasted shallots:
1) Preheat the oven to 200C/Gas Mark 6.
2) In a large bowl, toss the shallots with oil, and the salt and season with pepper to taste. Transfer to an aluminium foil-lined baking tray and arrange in a single layer. Bake until the shallot skins are very tender, about 40 mins. Remove from the oven and set aside to cool.
3) Using a knife, slice off the tips of the shallots and discard. Gently squeeze the shallots from their skins and season with salt and pepper to taste. Serve warm or at room temperature.