Marinade (optional):

1. In a relatively shallow bowl, mix garlic, bay leaf, olive oil, balsamic and pepper to taste.
2. Add steak, turn to coat and cover it with plastic wrap.
3. Refrigerate until ready to use, at least 4 hours, turning it occasionally.

Without marinade:

1. Cut the room-temperature sirloin into two equal pieces. 
2. Season the meat with coarse salt and freshly ground black pepper (preferably).
3. Let stand for 10 minutes before cooking. 

For both methods:

1. Heat 2 tablespoons olive oil (or butter or 1 tablespoon each) in a large skillet. 
2. Add meat and cook over medium heat for 4-5 minutes on each side, depending on the thickness of the meat and how juicy you like it. It can vary a lot.
3. Transfer to a plate and rest for 5 minutes to allow the juices to re-distribute.

To assemble the sandwiches:

1. Cut the rolls in half. Lightly toast them if you want to. I don't if using fresh bread. 
2. Cut the steak across the grain and, dividing evenly, put the slices over half of the bread, making sure no juices are left behind. The bread must be embedded in meat juice to get a tastier sandwich. 
3. Top the meat with a few tablespoons of chimichurri, add a little olive oil and top with tomato or onion slices if using (salt the tomatoes if needed).
4. Close the sandwiches with the remaining bread.
5. Eat immediately.