In the south of France, bakers make rolls specifically for this street-food favorite. Any large, crusty roll with a soft interior is suitable.
In a bowl, combine beans, onion, fennel, tomatoes, pepper, olives, capers, tuna, and watercress with 3/4 cup of the vinaigrette. Season with salt and pepper.
Slice open rolls, and scoop out some of the top halves. Brush remaining vinaigrette on insides of rolls. Pile salad mixture on bottom pieces, and top with egg slices. Cover with tops of rolls; serve.