Defrost the duck according to the packaged instructions, and remove the internal giblets.
Save the giblets, except for the liver. You can use the giblets to make bone broth or a poultry-based gravy.
Cook the liver in a small frying pan as a cook's treat! It's very nutritious.
Preheat the oven to 425°F (218°C).
Using the tines of a sharp carving fork or a skewer, pierce the skin of the duck all over. Do this at a 45-degree angle, and take care to pierce only the skin and not the duck's meat. (See video.)
Mix the coarse sea salt, black pepper, and red pepper flakes (if using) together, and rub this mixture in the duck's cavity.
Stuff the duck with garlic and oranges.
Mix the fine salt and the paprika together, and rub this mixture over the entire skin of the duck.
Place the duck breast-side down on the roasting rack in the roasting pan.
Place roasting pan into the preheated oven, close the oven door, and immediately lower the temperature to 350°F (177°C).
After one hour of roasting time, remove the roasting pan from the oven and place it on a heatproof surface.
Using two wooden spoons or other long-handled utensils, carefully flip the duck over so that it is breast side up.
Using a baster, remove any duck fat from the roasting pan that has been rendered. Strain out any bits and decant the fat. (See video.)
Return the roasting pan to the oven and continue to roast the duck for one more hour.
After the second hour, remove the roasting pan from the oven and place it on a heatproof surface.
If the bottom of the duck looks nicely browned, you do not need to flip the duck. You can continue to allow it to roast breast side up.
Using a baster, remove any duck fat from the roasting pan that has been rendered. As before, strain out any bits and decant the fat.
Return the roasting pan to the oven and continue to roast the duck for an additional one hour for a total of three hours until the internal temperature of the duck reaches 180°F (82°C). (Use a meat thermometer inserted at the thigh to measure the temperature of the duck. Be sure to avoid touching the bone so that you can get an accurate measurement.)
If you decide to rotate the duck again after the second hour of cooking, you will want to rotate it one last time, approximately 15-30 minutes before it is done. Be sure to finish roasting the duck breast side up.
Once the duck is fully cooked, remove the roasting pan from the oven and place it on a heatproof surface.
Spoon the orange marmalade glaze over the duck and return the duck to the oven.
Turn on the broiler and allow the glaze to begin to sizzle as the duck skin begins to become quite crispy. Watch this VERY closely. It will not take more than a minute or two.
Remove the roasting pan from the oven and place it on a heatproof surface. Transfer the duck to a serving platter and allow it to rest for about 10 minutes before slicing.
Roast duck is best enjoyed when it is initially cooked and served. However, leftovers can be stored in the refrigerator, well wrapped, for 1-2 days. Roast duck is best rewarmed in a skillet on the stovetop with a bit of water added.
If you want to freeze any leftover duck, be sure to wrap it well and use it up within a few weeks. Technically, it can be stored in the freezer for up to 3 months, but the texture of the cooked duck does begin to deteriorate.