Dressing

• 120 milliliters mayonnaise
• 6 grams coarsely chopped fresh dill
• 80 milliliters whole-grain Dijon mustard
• 1 tablespoon cider vinegar
• 1 tablespoon granulated sugar

• Kosher salt
• Freshly ground black pepper
• ½ English cucumber, about 170 grams, unpeeled, cut into matchsticks with a knife or with a plastic V-slicer or mandoline
• 1 Granny Smith apple, cored, unpeeled, and cut into matchsticks with a knife or with a plastic V-slicer or mandoline
• 4 salmon fillets (with or without skin), each 170 to 225 grams and about 3 centimeters thick, pin bones removed
• 4½ tablespoons extra-virgin olive oil, divided
• 4 thin slices red onion
• 8 slices country-style bread