Directions

1. Preheat oven to 475°F with rack in middle position. Line a large rimmed baking sheet with foil. Combine halved potatoes, 1 tablespoon oil, ½ teaspoon pepper, and ⅛ teaspoon salt in a large bowl: toss until well coated. Spread onto the prepared pan, cut-side down; reserve the bowl. Roast until starting to turn golden brown and crispy, about 10 minutes.
2. Meanwhile, toss halved broccolini, 1 tablespoon oil and ⅛ teaspoon salt together in the reserved bowl. Remove the potatoes from the oven and flip; push to one side of the pan and spread the broccolini into an even layer on the other side. Continue to roast until the potatoes are almost tender and the broccolini is bright green and crisp-tender, 8 to 10 minutes more.
3. While the vegetables are roasting, whisk ¼ cup mustard, 2 tablespoons maple syrup, ½ teaspoon smoked paprika and the remaining 2 tablespoons oil, ½ teaspoon pepper and ¼ teaspoon salt together in a small bowl. Transfer 2 tablespoons of the mustard mixture to another small bowl and whisk in 1 tablespoon orange juice.
4. Remove the vegetables from the oven. Push the broccolini to one side of the pan; toss the potatoes and spread on the other side. Place 4 salmon fillets in the center of the pan; spoon about 1½ tablespoons mustard mixture from the first bowl over the salmon, spreading with the back of a spoon to drip over the sides. Place in oven and increase oven temperature to broil. Broil until the salmon is just cooked through (125°F), the sauce is bubbling around the edges, and the broccolini is charred in spots, about 8 minutes. Drizzle the reserved mustard-orange juice mixture over the potatoes and broccolini and sprinkle with 1 tablespoon chopped tarragon.