Step 1 Heat the oven to 425 degrees. Rinse the cavity of the duck well under cold running water until the water runs clear. Set aside all giblets but the liver (discard the liver) for making stock. Pat the duck dry and rub about 1 tablespoon of the olive oil on the outside of the duck. Season with salt.
Step 2 In a roasting pan, toss the carrots, onion, celery and thyme with the remaining 1 tablespoon olive oil. Place the duck on top of the vegetables; roast for 15 minutes. Reduce the oven temperature to 350 degrees and continue to roast an additional 1 hour to 1 hour, 20 minutes until a thermometer inserted in the thigh reaches 145 degrees. (The meat will be pink.) Remove the duck from the oven and let stand until cool enough to handle.
Step 3For the duck fat, strain the roasting juices from the pan and skim the fat from the juices (you should have two-thirds cup fat for the crust). Place it in the freezer until the fat completely solidifies, about 2 hours. Once the roasted duck has cooled, remove skin and fat. Tear the skin into pieces and place it in a heavy-bottomed saucepan. Cook over low heat to render the fat, about 1 1/2 hours. Strain fat and set aside. You'll need about a one-fourth cup fat for the roux for the filling. If you're short of fat, you can add butter.
Step 4Meanwhile, remove the duck meat from the carcass; set aside the carcass for stock. Shred the meat into bite-size pieces. Cover and set aside.
Step 5For the stock, scrape the roasted vegetables along with any drippings into a saucepot. Add the reserved giblets along with the duck carcass from roasting the duck. Add 10 cups water, black peppercorns and 1 teaspoon salt. Bring to a boil. Reduce the heat and simmer about 1 1/2 hours. Skim off the excess fat as the stock cooks. Remove from the heat and strain the stock through a fine mesh strainer. Rinse the saucepot and pour the strained broth back into the pan. Bring to a boil and reduce until the stock measures about 4 cups.
Step 6Prepare the vegetables. In a clean pot, blanch the kale in boiling salted water about 1 minute, drain well and set aside. In a saute or frying pan, cook sliced carrots and turnips in 1 tablespoon butter over medium-high heat until just tender, 7 to 8 minutes.
Step 7For the roux, heat one-fourth cup duck fat in a small saucepan until hot. Stir or whisk in one-third cup flour and cook, stirring constantly, until smooth and bubbly. Simmer 2 to 3 minutes, then stir the roux into the hot stock. Heat and stir until the sauce is smooth. Bring to a boil over high heat, then reduce to a simmer. Season to taste with salt. Cook until the sauce thickens, about 5 minutes.
Step 8Stir the shredded duck along with sauteed vegetables and the kale into the sauce. Cover and keep warm.