In a small mixing bowl, whisk together the red wine vinegar, mustard, salt, and pepper. While continuing to whisk, gradually add the olive oil. Whisk until an emulsion forms. Set aside.
Slice the baguette horizontally into 2 pieces. Tear out some of the soft bread in the center of each side, making a slight well in the bread. Place the tuna, green pepper, red onion, hard-boiled eggs, olives, and tomato on the bottom side of the bread in that order. Drizzle the vinaigrette over the vegetables, top with the second piece of bread, and wrap tightly in plastic wrap. Let stand at room temperature for 2 hours before serving.
Cut into 4 sandwiches and serve.
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