1 large strip, round, or shell sirloin steak, weighing between 12 ounces and 1 pound
1/2 t. coarsely ground black pepper
Salt to taste
2-3 T. oil
2 T. chopped onion
4 oz. crimini mushrooms, cleaned and chopped
1/2 t. thyme
1 loaf French or Italian bread, or 1 Vienna loaf
3-4 T. prepared mustard
Trim any excess fat from the steak. Sprinkle both sides with pepper and salt, then brush with a tablespoon of the oil. Heat a cast-iron or nonstick pan and sear the steak about 2 minutes on each side. Remove from the pan. Cook the onion for 2 minutes in the remaining oil, then add the chopped mushrooms, sprinkling them with the thyme and a little salt. Cook, stirring, until they have softened. If the mixture dries out, add a tablespoon or so of water.
Cut a large piece of bread about 1 inch longer than the steak. (If you’re using a Vienna loaf, you may not need to cut it at all.) Slice it lengthwise through the middle, leaving an uncut edge as a hinge. Spread both cut sides of the bread liberally with mustard. Now place the steak on the bottom half and scatter the mushroom mixture on top. Fold the top half of the bread down. Wrap the sandwich in two layers of plastic wrap. Take a long piece of string and tie the sandwich up tightly as if it were a parcel, knotting the string in several places. Put the sandwich on a plate and place another plate on top of it. Now weight it down with either cans of food or a bowl filled with water. Leave it for 8 hours or overnight in the refrigerator or other cold place. When you rescue it from the weights, it will be firm and flat. For serving, unwrap it and slice it on the diagonal so you have several strips, each about 3/4 to 1 inch wide. Arrange these on a platter with a bowl of relish or chutney to accompany them. For outdoor eating, leave the sandwich in its package.