Directions

1. Cover potatoes with 2 inches of cold water in a medium saucepan. Bring to a boil. Reduce heat to medium-low; simmer until potatoes are tender, about 5 minutes. Drain potatoes, and let cool completely; set aside.
2. Prepare an ice-water bath; set aside. Fill saucepan with cold water, and bring to a boil. Add green beans; cook until bright green, about 1 minute. Using a slotted spoon, transfer green beans to ice-water bath to stop the cooking; let cool completely. Cut green beans into 1/2- inch pieces; set aside.
3. Refill ice-water bath; set aside. Fill a small saucepan with cold water; add egg. Bring to a boil. Remove from heat; cover, and let stand 11 to 12 minutes. Transfer egg to ice-water bath; let stand 2 minutes. Peel egg, and cut in half; discard yolk. Cut egg white into 1/2-inch pieces, and set aside.
4. Whisk together mustard and vinegar in a small bowl. Whisk in oil in a slow, steady stream; set vinaigrette aside. Put potatoes, green beans, egg, olives, shallot, and tuna into a large bowl. Sprinkle with parsley and salt; season with pepper. Drizzle vinaigrette over tuna mixture; gently stir to combine.
5. Using your hands, hollow out 1 half of each baguette piece, leaving a thin layer of bread. Arrange tomato slices over remaining half. Spoon 1/2 cup tuna salad onto hollowed-out half, and top with tomato-lined half.