In a large pot of boiling salted water, cook green beans until bright green and tender, 3 minutes. Drain, rinse under cool water, and chop. In a medium bowl, whisk together mustard, vinegar, shallot, and olive oil. Drain tuna, then break into chunks and add to vinaigrette, along with green beans. Season to taste with salt and pepper and stir to combine. In a food processor, pulse together tomatoes, olives, and parsley. Split baguette lengthwise and scoop out most of the insides. Arrange a few lettuce leaves on bottom half; top with tuna mixture and eggs. Spread tomato-olive mixture on top half of baguette and place on sandwich. To serve, cut crosswise into 4 pieces.