Tuna Melts with Quick-Pickled Fennel

Ingredients

• 1/2 tsp. freshly ground black pepper
• 1 tsp. Dijon mustard
• 1/2 cup rice vinegar
• 1 1/2 tsp. kosher salt (divided)
• Zest and juice of 1 lemon
• 1 cup freshly grated mozzarella cheese
• 2 Tbsp. chopped fresh parsley
• 1 1/2 Tbsp. granulated sugar
• 1 small fennel bulb (thinly sliced, plus 1 Tbsp. chopped fennel fronds)
• 2 5-oz. cans tuna in water, drained
• 3 Tbsp. mayonnaise or whole-milk Greek yogurt
• 2 tsp. capers (drained)
• 5 slices whole-grain bread