Preheat the oven to 475 F. Arrange the rainbow roasting potatoes, if using, in the bottom of a shallow roasting pan. In a small bowl, whisk together the dried marjoram, thyme, parsley, and cumin.
Gently score the skin of the duck in a crisscross pattern, taking care not to slice into the meat. Pat the duck completely dry then rub in the herb mix so that every part of the duck is covered in herbs.
Quarter the onion and used blood orange rinds. Place them into the cavity of the duck. Secure the tail up between the legs, and tie with cooking twine to keep the cavity closed. Place the duck into the oven and bake for thirty minutes.
While the duck cooks, squeeze the blood oranges and collect the juice in a small bowl. Whisk in the white wine vinegar, hot honey, orange juice and chicken broth.
Remove the duck from the oven and reduce the temperature to 350 F. Tilt the pan at a slight angle, taking care to secure the duck, and pour off any excess fats in the roasting pan.
Pour the blood orange and broth mixture onto the duck and into its cavity. Place back into the oven and bake for an hour to an hour and a half. **Every 20-30 minutes, tilt the pan to drain excess fat and juices.**
Let the duck cook for about fifteen minutes before starting the l'orange sauce. In a small bowl, whisk together the remaining orange juice, ginger, and white wine vinegar.
In a large sauce pot over high heat, melt the sugar until caramelized. To do this, pour in the sugar and let set until crumbly, stirring occasionally. As the sugar continues to melt, stir more frequently so it doesn't burn. Stir until the sugar has turned into a liquid caramel.
Pour in the orange juice mixture. It will sputter and steam, so do this VERY CAREFULLY. Continuously stir the sauce until the caramel has melted into the liquid to form a thick sauce.
Pour out the excess juice, but this time, pour it into a large pour or jar. Let the fats set and then skim off the golden liquid layer on top. You will need about 1/4 cup liquid.
Whisk the flour into the golden liquid until mostly incorporated. Pour the mixture into the orange caramel sauce. Continue to stir until the sauce is smooth and glossy.
The duck will be about 120 F - 130 F internally at this point - not quite ready (internal temperature will be 135 F for a rare duck, 140 F for medium, and 160 F-170 F for well done). Remove the duck from the oven.
Pour half of the l'orange sauce onto the duck so it is completely coated. Return to the oven for ten minutes, then pour on the remaining sauce. Cook for another 15-30 minutes, or until the duck has reached 135 F (or your desired doneness). Serve with a kiss, and enjoy!