Salmon in Parchment Paper with Horseradish-Yogurt Sauce

Ingredients

• 1 teaspoon mustard seeds
• 1 bunch kale, center ribs and stems removed, torn into 2” pieces, or 1 flat-leaf spinach, thick stems removed
• 8 oz. baby Yukon Gold potatoes (about 6), unpeeled, very thinly sliced
• 2 medium shallots, thinly sliced
• 4 6-oz. skinless salmon fillets
• Kosher salt and freshly ground black pepper
• 1 lemon, very thinly sliced, seeds removed
• 2 tablespoons olive oil
• ½ cup plain yogurt
• 2 tablespoons chopped fresh dill
• 1 tablespoon Dijon mustard
• 1 tablespoon prepared horseradish
• Kosher salt and freshly ground black pepper

Steps

1. Preheat oven to 400°. Toast mustard seeds in a small dry skillet over medium heat until golden and some begin to pop, about 1 minute. Transfer to a small plate and let cool.
2. Lay 4 large rectangular pieces of parchment paper on a work surface. Place kale off-center on each sheet; top with potatoes, shallots, then salmon; season with salt and pepper. Top fish with lemon slices and mustard seeds; drizzle with oil.
3. Fold parchment over fish and crimp edges tightly to form a sealed packet. Place packets on a large rimmed baking sheet. Bake fish 20 minutes.
4. Salmon packets can be assembled 6 hours ahead; chill.
5. Whisk yogurt, dill, mustard, and horseradish in a small bowl. Season with salt and pepper.
6. Carefully open salmon packets and serve fish with horseradish-yogurt sauce.
7. DO AHEAD: Yogurt sauce can be made 8 hours ahead. Cover and chill.