Season the duck with Creole seasoning and cut shallow slits in the skin. Place it on a roasting pan with a rack and into the 500-degree oven. Immediately lower the oven to 350 degrees. Roast until a meat thermometer inserted into the breast reads 150 degrees. (This is underdone, intentionally.) Remove.
After the duck has cooled enough to handle, strip the meat off the bones in pieces as large as possible. Reserve the skin and the bones separately.
Make the sauce. Heat the olive oil in a large, heavy Dutch oven over high heat until it shimmers. Saute the onion, garlic, carrot, celery, bay leaves, peppercorns, thyme, and rosemary until the onions are lightly browned. Then add the duck bones (broken up enough so that they lie flat in the pan) and the tomato sauce. Lower the heat to medium and saute another five minutes.
Add the beer and bring to a light boil, scraping the bottom of the pan as you do. Add a quart of water (or, better still, chicken or duck stock) and return to a simmer. Reduce until the sauce coats the back of a spoon.
Strain the sauce, pressing to get all the juices. Add salt to taste.
In a medium skillet, heat the butter and saute the onions and mushrooms till the onions are transparent. Add the duck meat and saute until medium rare to medium. Add 2 cups of the sauce and bring to a boil. Simmer for additional two minutes.
Serve topped with a sprinkling of green onions and crushed peanuts.