This is a great sandwich. Steak sandwiches should be, after all. And it is really easy to prepare because both the steak and sauce can be made in advance. Shell steak is as much sought after as filet, but it is not as expensive. Its robust flavor is complemented here by the emphatic taste of capers. If you want to allow your guests to assemble their own sandwiches, place the steak on a platter, the sauce in a bowl, and the bread in a napkin. Then watch as it all disappears. A lush, big green salad is always a good accompaniment.
Preheat the oven to 400 degrees Fahrenheit. Place the roasting pan in the upper third of the oven to preheat, as well.
Roast the steak: Season the steak on both sides with salt and pepper. Carefully place on the preheated roasting pan. Roast for 8 minutes. Turn the steak with a spatula and roast for 8 minutes more. (It will be medium.) Remove from the oven and let cool to room temperature.
Make the caper sauce: Peel, trim, and thinly slice the onion and garlic. Trim and dice the celery stalks, including some of the leaves. Wash, trim, and cut the tomatoes into small chunks.
In a medium skillet, sauté the onion, garlic, and celery in the olive oil over medium heat, stirring, until the onion is limp. Add the wine and cook until it evaporates. Add the tomatoes and salt and pepper. Reduce the heat to low, cover, and cook for 10 minutes more. Stir in the capers, cover, and cook for 10 minutes more. Remove the pan from the heat and let the sauce cool. Add the parsley and stir until well combined.
Holding a carving knife at an angle, cut the steak into thin slices on the bias on a cutting board.
To assemble the sandwiches, arrange 6 of the bread slices on the counter in front of you. Top them with slices of the steak, and then a generous amount of the caper sauce. Cover each one with a slice of the remaining bread and cut in half.