Step 1Pick over the lentils to remove any stones. Rinse well in a fine sieve under cold running water. Place in a medium saucepan. Add the leek, garlic, bay leaves and 1 teaspoon salt.
Step 2Cut the carrot in half crosswise, then lengthwise and add to the pot. Add cold water to cover by 2 inches.
Step 3Bring to a boil, stirring often. Reduce the heat to very low and simmer, stirring occasionally, until the lentils are just tender but still firm, 17 to 20 minutes. Do not overcook. Remove from heat and drain well.
Step 4While the lentils cook, heat the oven to 500 degrees. Whisk together the mustard and vinegar in a small bowl. Whisk in the hazelnut oil until emulsified.
Step 5Discard the bay leaves, garlic and carrot from the cooked lentils. Combine the lentils and all but 1 tablespoon of the vinaigrette in a shallow bowl, mixing well. Set aside in a warm spot.
Step 6Lay the duck legs on a foil-lined broiler pan. Cook them under the broiler 6 inches from the heat source, turning once, until the skin is well crisped and the meat is warmed through, about 10 to 15 minutes. Using a fork and knife, shred or chop the meat and skin into rough pieces, trimming excess fat.
Step 7Add the meat to the lentils and mix well. Add the chives and tarragon and salt and pepper to taste.
Step 8To serve, toss the frisee with the remaining 1 tablespoon vinaigrette and distribute it among 4 salad plates. Top with the lentil mixture. Sprinkle with hazelnuts.