Preheat oven to 375 degrees. Score the duck skin with a knife, then place duck legs in a small roasting pan or casserole dish. Sprinkle evenly with salt and pepper. Add wine, garlic and rosemary, turning the duck legs a few times to evenly coat, ending with the skin side up. Roast for 90 minutes, turning twice.
While the duck roasts, make the other accompaniments. In a medium skillet, cook the onions with butter, salt and pepper on medium heat, stirring occasionally, until caramelized, about 30 minutes. Set aside.
Bake the flatbread on a large baking sheet for 7-10 minutes or until crispy and light golden brown. (This can be done while the duck cooks, simply use the second rack in your oven.)
In a small bowl, mix together the Roquefort and sour cream.
When the duck has finished cooking, remove the legs from the pan and shake off excess liquid. Reserve 1/2 cup of the cooking juices in a small bowl. When the duck is cool enough to handle, shred the meat and skin into bite-sized pieces.
To assemble, add equal amounts of arugula, onions, duck and roquefort to half of the flatbread slices, drizzle with reserved cooking juices, then top with remaining flatbread slices. Serve immediately.