In a large, heavy-bottomed skillet, heat the olive oil over medium-high heat and add the onions. Season the meat with salt and pepper, to taste, and add to the pan. Cook over high heat, stirring occasionally, until the meats are browned. Add the wine and let it evaporate, then add the tomato paste, canned tomatoes, and red pepper flakes, cover the pan, and cook for 2 1/2 hours over low heat. Stir in the parsley and keep hot. Remove the duck legs and remove them from the bones. Shred the meat with a fork and return to the pan.
Bring 6 quarts of water to a boil over high heat and add 2 tablespoons salt. Cook the fusilli in the boiling water until tender yet al dente, about 10 minutes. Drain, reserving 1 cup of the pasta cooking liquid, and add the pasta to the meat sauce, thinning with the pasta water, if necessary. Toss over high heat 1 minute, then divide evenly among 4 pasta bowls. Top with grated cheese and serve.