In a medium bowl, whisk together olive oil, vinegar, and mustard and season with salt and pepper. Toss tuna with the dressing.
Cut bread in half horizontally and remove most of soft interior crumb (you can save and freeze it for croutons).
Spread olive tapenade on bottom half of bread. Top with basil leaves and egg slices. Season with salt and pepper. Top with tuna, followed by cucumber and onions.
In a small bowl, mash together chickpeas and garlic together. Spread chickpea mixture on other half of bread, top with avocado, and season with salt and pepper.
Close sandwich and wrap tightly in plastic wrap. Place sandwich between two baking sheets and place a heavy skillet on top for weight.
Let rest for about 1 hour (or refrigerate, up to overnight).
To serve, cut into quarters.