Prepare the barbecue (medium-high heat). Grill the baguette pieces, cut side down, until lightly toasted, about 2 minutes. Remove and set aside.
Combine the olives, capers, 1 tablespoon of oil, lemon juice, parsley, lemon zest, and anchovy in a food processor. Pulse just until the olives are finely chopped. Season the tapenade with salt and pepper.
Drizzle the tuna with the remaining oil. Sprinkle with salt and pepper. Grill the tuna until golden and seared on the outside, and just opaque in the center, about 2 minutes per side.
Stir the mayonnaise and mustard in a small bowl to blend. (The tapenade and Dijon-mayonnaise can be prepared 1 day ahead. Cover separately and refrigerate. Bring the tapenade to room temperature before using.)
Spread the Dijon-mayonnaise over the bottom and top halves of the baguette pieces. Top with red peppers and red onion. Cut the tuna steaks across the grain into 1/2-inch thick slices. Arrange the tuna slices atop the onions. Spoon the tapenade over the tuna. Top with the lettuce then the baguette tops, and serve.