Make tuna salad: Whisk together mayonnaise and lemon juice in a large bowl. Add remaining salad ingredients and stir together gently. Season with salt and pepper.
Assemble sandwiches: Cut baguette into 4 equal lengths and halve each piece horizontally. Brush cut sides with oil and season with salt and pepper. Make sandwiches with baguette, lettuce, and tuna salad.
Recipe courtesy of Gourmet Magazine