- The day before, place beans in a large bowl. Cover with water and let soak overnight at room temperature. Add water, if needed, so that beans are always covered. Rinse and drain.
- In a saucepan, brown onions in butter. Add broth, mushrooms and beans and bring to a boil. Cover and simmer over medium heat for about 1 hour. Remove lid and continue cooking for about 30 minutes or until beans are very tender. Season with salt and pepper. Set aside.
- With a sharp knife, remove most duck fat so only a thin layer remains. Score fat with a knife, without touching meat. Season with salt and pepper.
- In a skillet, brown duck over low heat, fat side down, for about 10 minutes or until fat is golden brown and crisp. Turn duck meat and cook until desired doneness. Let rest on a plate for about 10 minutes.
- In the same skillet, wilt spinach in melted duck fat. Season with salt and pepper.
- If you wish to serve your dish with foie gras, in a hot non-stick skillet, brown slices over high heat on both sides until the centre of the liver is soft, about 1 minute on each side. Season with salt and pepper. Be careful not to overcook, as foie gras will melt like butter. Drizzle beans with melted fat.
- Thinly slice duck and serve with spinach, beans and drippings. Top with foie gras.