Peel and trim 1 1/2 pounds carrots. If they are more than 1-inch thick, cut them in half lengthwise; otherwise, leave them whole. Cut the carrots crosswise into 2-inch-long pieces. Thinly slice 4 medium scallions, keeping the white and green parts separate. Pat 4 salmon fillets dry with paper towels, then season on both sides with 1 teaspoon of the kosher salt and 1/4 teaspoon of the black pepper.