In a small mixing bowl, whisk together the red wine vinegar, agave, mustard, salt, and pepper. While continuing to whisk, gradually add the olive oil. Whisk until the oil and vinegar are emulsified.
Slice the baguette horizontally into 4 pieces or slice the individual buns horizontally.
Place the lettuce, tuna, hard-boiled eggs, olives, cucumbers, tomato and red onion on the bottom side of the bread in that order. Drizzle the vinaigrette over the vegetables and tuna, top with the second piece of bread.
Serve the 4 sandwiches or cut into 4 sandwiches if using a baguette.