Place the brie on a baking sheet and slowly melt in the oven.
Bring a pot of water to a boil and heavily salt it (it should taste like the sea). Trim the ends of the string beans and add to the boiling water. Allow the water to come back to a boil, then blanch for 1 minute, so they are still slightly raw. Immediately drain. The string beans will continue to cook a little.
Add the tuna to a medium mixing bowl. Thinly slice the red onion using a mandolin and macerate in the cornichon brine for 5 minutes. Mince the olives and cornichons and add to the tuna. Zest half of the lemon zest and juice the whole lemon and add to the tuna. Chop the macerated onions and green beans and add to the tuna. Add the Dijon mustard and sweet relish and mix until well incorporated.
Cut the baguette into your preferred serving size. Toast in the oven or grill until golden brown. Hollow out the inside of the bread so you can pack in as much tuna salad as possible.
Spoon the tuna salad onto the bread. Place a handful of green beans on top. Remove the brie from the oven and hold the pan above the sandwich. Use a butter knife or icing spatula to scrape some cheese off the edge of the rind onto your sandwich. Garnish with chives.
This recipe was created by a contestant during a cooking competition. It has not been tested for home use.