1/3 cup pitted Castelvetrano olives 1 tablespoon extra-virgin olive oil 2 teaspoons lemon zest, plus 2 Tbsp. fresh lemon juice, divided 2 tablespoons fresh chopped parsley 1 tablespoon roasted sunflower seeds 1/4 teaspoon kosher salt, divided 1/4 teaspoon freshly ground black pepper, divided 1/4 teaspoon crushed red pepper flakes 1 (5-oz.) can solid white albacore tuna in water, drained 2 tablespoons fresh chopped dill 2 tablespoons mayonnaise or plain whole-milk Greek yogurt 1 teaspoon Dijon mustard 1 1/2 ounces grated low-moisture mozzarella cheese 4 thick slices whole-grain bread