Peel the onions and slice them in half from top to bottom. Trim the top and bottom of each half.
Cut the onion halves into chunks by cutting at right angles to the original cut. Make the pieces as consistently sized as possible so they will cook evenly.
Set a large saucepan on the stove over medium-high heat and let it warm up. Add the 60ml (4 tbsp) butter to the pan. When the butter melts and begins to bubble, add the onions. Stir the onions so that they are well coated. Cook for about 5 minutes or until the onions begin to sweat (look glossy).
Add the salt.
Continue cooking on low heat for about 15 to 20 minutes until the onions have softened, stirring occasionally.
Take the pan off the heat and serve warm. If making ahead, allow the onions to cool and then store them in an airtight container in the fridge.
Halve and butter each baguette.
Pile on the lettuce, steak and onions and close the baguettes with their other halves. Serve and enjoy!
Grated cheese, melted or not works well with this sandwich. Ketchup or mayonnaise too!