Grilled Pork Tenderloin Sandwiches

Name: Grilled Pork Tenderloin Sandwiches
Ingredients: 1 oil-packed anchovy fillet; Kosher salt; 1 cup mayonnaise; 3 tablespoons fresh lemon juice; 2 garlic cloves, finely grated; ¼ cup chopped basil, plus more for serving; 2 tablespoons chopped chives, plus more for serving; Freshly ground black pepper; 2 tablespoons fennel seeds; 1 teaspoon crushed red pepper flakes; 2 tablespoons Diamond Crystal or 3½ teaspoons Morton kosher salt, plus more; 2 pork tenderloins (about 1½ pounds each); Vegetable oil (for grill); 1 ciabatta loaf, halved lengthwise; 6 cups mature arugula leaves with tender stems; Torn basil leaves, chopped chives, and lemon wedges (for serving); A spice mill or mortar and pestle