Imagine the classic French salad but in a fun, portable sandwich! This specialty of Nice is known as pan bagnat ("bathed bread"). In this version, tuna is tossed in a creamy dressing with Dijon mustard and fresh thyme, then piled onto a demi baguette with some fresh vegetables and olive tapenade. Et voila! It's like you're in the south of France.
Bring a small saucepan of water to a boil over high heat. Gently lower the eggs into the boiling water with a slotted spoon. Reduce the heat to medium-high and cook for 10 minutes, until both the whites and yolks are fully set (hard-boiled). Transfer to a large bowl filled with ice water to stop the cooking. Peel the eggs, then thinly slice.
Meanwhile, place the red onion in a small bowl and cover with ice water. Soak for 5 to 15 minutes (this will remove some of the sharp, raw onion flavor). Whisk together the mayonnaise, mustard, vinegar, thyme, 1/4 teaspoon salt and a few grinds of pepper in a medium bowl until smooth. Drain all of the oil from the tuna fillets, then break up into bite-size pieces. Fold the tuna into the dressing until evenly coated.
Cut the demi baguette in half crosswise, then halve each piece lengthwise to create 2 sandwiches. Spread the tapenade on the inside of the sandwich tops. Layer the tomato, red onion, egg and tuna on each sandwich bottom, dividing evenly. Top with the cucumber and spring greens, then close the sandwiches. Wrap each sandwich in parchment and refrigerate until ready to serve, up to 4 hours. Alternatively, cut each sandwich in half and serve immediately.