Lemon-Caper Tuna Sandwich

Ingredients

• 1⁄4 cup rice wine vinegar
• 3 tbsp. sugar
• 1 tsp. kosher salt
• 1 fennel bulb, trimmed and cut into 1/4" pieces
• 1⁄4 cup capers, drained and minced
• 6 tbsp. olive oil
• 2 tbsp. minced chives
• 1 1⁄2 tbsp. Dijon mustard
• 2 (4 oz.) cans tuna in water, drained
• 2 lemons, zested and juiced
• 2 sprigs rosemary, minced
• Kosher salt
• freshly ground black pepper
• 1⁄2 Whole-wheat baguette, halved lengthwise and then crosswise
• Baby arugula, for serving
• Olive tapenade, for serving