Tuna Nicoise Sandwich

In a medium bowl, whisk together oil, vinegar, and mustard; season with salt and pepper. Transfer 2 tablespoons dressing to another bowl; toss with tuna. To remaining dressing, add cucumber and onion; toss to combine.

Cut bread in half horizontally; remove most of soft interior crumb. Spread tapenade on bottom half. Top with basil leaves, then egg slices; season with salt and pepper. Top with tuna, then cucumber mixture; close sandwich.

Wrap sandwich tightly in plastic and place between two baking sheets. Weight with a heavy skillet. Let stand 1 hour (or refrigerate, up to overnight). To serve, cut into quarters.