How to Make It
Slice baguette in half lengthwise without cutting all the way through. Brush bottom half with 1 tablespoon oil and arrange basil leaves on top. Set aside.
Whisk together vinegar, mustard, salt, and black pepper in a large bowl. Add onion and toss to combine. Set aside to marinate, about 10 minutes. Remove onion from bowl and arrange on top of basil. Reserve vinegar mixture.
Slowly whisk remaining ￼ cup oil into vinegar mixture. Add tuna, green beans, and olives. Toss and add mixture to baguette. Top with red peppers and parsley and close baguette.
Wrap baguette tightly with plastic wrap and place between two baking sheets. Place a heavy skillet on top of baking sheet to compress.
Chill, pressed, for at least 1 hour and up to 4 hours. Unwrap and slice crosswise into 4 sandwiches.