Preheat oven to 425°F. Trim leeks, removing and discarding dark green parts. Remove and discard outer layer of each leek; cut into 1-inch coins.
Toss the leeks with 1 tablespoon oil, 2 teaspoons thyme, and ½ teaspoon pepper on a large rimmed baking sheet until well coated. Spread into an even layer around the edges of the pan. Roast until mostly tender and starting to brown, about 10 minutes. Drizzle with ½ cup broth and 1 tablespoon lemon juice; roast until the liquid has been fully absorbed, about 12 minutes.
Meanwhile, stir 2 tablespoons mustard and 1 tablespoon tarragon together in a small bowl; spread evenly over 4 salmon fillets (about 1½ tablespoons each). Combine ½ cup panko, 1 tablespoon each parsley and oil, 1 teaspoon lemon zest and the remaining 2 teaspoons thyme in a medium bowl. Sprinkle evenly over the salmon, pressing gently to adhere.
Carefully flip the leeks. Drizzle the center of the baking sheet with 1½ teaspoons oil and place the salmon there. Roast until the leeks are well browned and the salmon is just cooked through, about 12 minutes. Sprinkle the leeks and salmon with ⅛ teaspoon salt.
Meanwhile, prepare lentils according to package directions. Combine ¾ cup yogurt, ½ cup spreadable cheese, grated garlic and the remaining 1 tablespoon tarragon, 2 tablespoons parsley, 1 teaspoon lemon zest and ½ teaspoon pepper in a bowl. Add the remaining 1 teaspoon oil and ⅛ teaspoon salt to the lentils; toss to coat. Spread the yogurt mixture on 4 plates; top with the lentils, leeks and salmon. Garnish with additional tarragon, if desired.