Make the vinaigrette: Cut off and discard ends of lemon. Cut lemon into quarters; remove and discard seeds. Transfer lemon to a food processor or blender, and process until finely chopped, about 1 minute. Add oil, herbs, vinegar, mustard, and garlic; process until mostly smooth and well combined, 30 seconds to 1 minute. Transfer vinaigrette to a small bowl, and whisk in salt and pepper.
Make the salad: Stir together yellowfin tuna, ¾ cup vinaigrette, capers, and celery in a medium bowl. Serve tuna salad on toasted bread with mayonnaise or with crackers or toasted baguette.
