Mix tapenade, onion, basil, 5 tablespoons oil, vinegar, and lemon peel in medium bowl; season with salt and pepper.
Brush baguette slices and tuna on both sides with oil. Sprinkle tuna with salt and pepper. Heat large nonstick skillet over medium-high heat. Add baguette slices; toast until golden, 3 to 4 minutes per side. Transfer to plate. Add tuna to same skillet; cook to desired doneness, 1 to 2 minutes per side for medium-rare.
Divide arugula among 4 plates. Cut tuna into slices; arrange atop arugula. Spoon tapenade vinaigrette over tuna. Place 2 baguette slices alongside each and serve.
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