These vegetable sandwiches are the perfect picnic food because they get better and better as the day wears on; the flavors continue to blend and the juices to soak into the bread.
Roast peppers by broiling them, skin side up, as close to the heat as possible, until skin is well browned. Place in a plastic bag until cool enough to handle, about 20 minutes; save any liquid they've given off. Peel peppers and discard skins. Cut peppers into long strips, saving any liquid. Place peppers and juice in a large bowl.
Coarsely chop parsley and watercress, and add to peppers along with olives, tomatoes, scallions, and capers. Add salt and pepper to taste. Add enough dressing to coat, and stir to combine.
Cut baguettes in half lengthwise, leaving a hingecon one side. Scoop out middle of loaves and discard. Leave a little bread near edges so the filling doesn't leak out.
Brush insides of bread shells generously with dressing. Pack marinated vegetables into baguettes and close up. Wrap tightly in plastic and let stand, covered with a cookie sheet weighted with heavy books, for 1 to 3 hours. Cut each baguette into 3 or 4 pieces and serve.