This isn't going to be worth making unless you've roasted a whole duck and gone to the trouble of making duck stock, picking all the little shreds of meat off the bone that are left after slicing the breasts and legs, and rendering the fat that comes off during roasting. BUT! If you do happen to do all of the above, this is an exceptional soup to cook up a month later when you're hungry for some of the same flavors but with a whole new twist.
My Christmas Eve dinner was a roasted duck, posted here. The next day I covered the carcass with water, including the apple and onion that had been stuffed inside the cavity, and simmered it down to a rich stock. I don't recall adding any herbs.
I had also saved the fat that came out of the duck during roasting. The recipe I used called for a pan sauce to be made from the drippings, but only after skimming the fat. I prudently saved the fat that seperated at the top of the drippings in a jar in the freezer, which I pulled out to start this soup. I'm not going to write this up as an actual recipe, because even if I make it again, it won't be following exact proportions, it will just be using whatever I have around, so that's how you should do it too.
I heated some of the reserved duck fat in the stock pot and used it to saute some chopped onion. Then I stirred in some dried lentils and added my duck stock. Cover the pan and simmer about 30 minutes, then add reserved duck meat shreds along with vegetables and 2-3 tablespoons tomato paste. I purchased two medium parsnips just for this purpose, because I think they taste a lot like carrots but are a little different, with a sweetness that I thought would complement the duck well. They were sensational! So dice them and don't skip them. I also had a leftover half a russet potato and half a sweet potato (moderately successful french-fry attempt) which I diced. Cover and continue to simmer until potatoes are easily pierced with a fork but not falling apart, about 20 minutes. Taste broth and season with salt and pepper if needed. I stirred in a huge handful of fresh parsley leaves, left mostly whole, and a teaspoon of chile-garlic sauce. Sprinkle on a little more parsley for color, but otherwise it doesn't need any garnish.
The sweet potatoes and parsnips are a great way to play off the traditional sweet combinations that you often find with duck (plum, cherry, etc.) and using duck in the soup is reminiscent of a turkey soup but rich and flavorful and different enough that it will really seem a special treat.