1Place duck legs fat-side down in a large skillet over medium-high heat, searing them on both sides until golden, about 3 minutes per side. Remove legs from pan, and reserve about 3 tablespoons of fat.
2Place onions in skillet with reserved duck fat, and cook over medium heat until they are soft and translucent, about 5 minutes. Stir in garlic, serrano peppers, and ginger; cook for about 3 minutes.
3Skim the coconut cream from 1 can of coconut milk, and stir the cream into the onion mixture. Stir in the curry paste, and cook until fragrant, about 1 or 2 minutes.
4Stir into the skillet the additional 2 cans of coconut milk, kaffir lime leaves, green onions, and fish sauce. Return duck legs to the pan. Simmer on low heat until just done, about 15 minutes. (If the curry sauce needs further reducing, remove the duck legs to an oven set at low heat, and cook down the sauce over medium heat.)
5Meanwhile, bring a large pot of lightly salted water to boil, and cook the noodles according to package directions. When done, drain, and run cold water over noodles. Set aside.
6Toss the noodles in the curry, and serve with chopped cilantro.