How to Build a Pan Bagnat, the Picnic-Perfect Sandwich
1. Hollow out the roll.
Tear the crumb out of the top and bottom halves of the boule to make shells about 1 inch thick. Save the torn crumb for other uses, such as a fresh bread-crumb topping or rustic croutons. Then toast the hollowed halves to help them stand up to the oil dressing, giving the bread texture like a good Thanksgiving stuffing: crisp yet moist.
2. Bathe the bread.
Pan bagnat roughly translates to "bathed bread" in French; the roll gets a light soaking in fruity oil then absorbs flavors from the fillings as it stands. Spoon the tuna mixture into the bottom roll half, mounding and pressing as needed to make it fit. Drizzle the top half of the roll with oil and lemon juice, and then layer the filling with the traditional toppers: fresh lettuce, tomato, and hard-boiled egg.
3. Wrap it up.
It's be fine to eat at this point, but pan bagnat gets even better with a little patience. Seal it snugly in plastic wrap and refrigerate two hours or overnight. This is when the magic happens: The layers press together, flavors marry, and textures grow more interesting. A tight wrap also helps the layers hold togetherso the thick sandwich is easier to slice.
4. Et voilà!
After resting in the fridge for hours, the sandwich hangs together nicely now, slicing into four impressively tall yet neat quarters. The bread shells have also taken on the flavors of olive oil and tart lemon, briny capers and olives, and pungent onion. If you take your pan bagnat to a picnic, leave it whole and wrapped. When ready to eat, just unwrap and slice to serve.