Pan Bagnat is a hollowed out roll filled with salad Nicoise. It is a very popular sandwich in the south of France. It can be made in advance and is easy to transport. If your rolls are the ready to bake sort then bake them according to the packet instructions and leave to cool completely before filling. This makes an excellent picnic recipe.
If using red onion then it’s a good idea if you can to soak it in iced water for 30 minutes to make it taste a little milder. Drain and pat dry with a clean tea towel or kitchen paper. Meanwhile, slice off the top of the rolls. Carefully scoop out the bread inside the rolls, leaving a wall of crust about 2 or 3 mm thick.
Put the tuna in a bowl. Whisk together the olive oil and lemon juice and season well with salt and pepper then drizzle the dressing over the tuna and toss gently. Put half of the tuna in the base of the rolls and pack it down tightly. Add a layer of egg and press down as firmly as possible without crushing the egg. Add a layer of sliced tomato sliced onion, and then the olives (if using). Finally add the remaining tuna and press down the filling gently so that the filling is just inside the roll. Spoon on any dressing from the tuna that may be left in the bowl.
Replace the lid of the rolls and wrap each one tightly in plastic wrap. Refrigerate for a minimum of 2 hours and up to overnight if you have the time. To serve, unwrap the rolls and cut each one in half. To serve, unwrap the rolls and cut each one in half. They are quite filling so each roll should serve two people and when you cut them in half you will see all the layers.