Inspired by German celebratory harvest meals from centuries past, this comforting braised dish trades the traditional goose leg quarters for easier-to-source duck, served on a bed of buttery-crisp spaetzle and saucy mushrooms.
Sprinkle duck legs evenly with salt and pepper. Arrange legs, skin sides up, on a wire rack set inside a baking sheet. Refrigerate, uncovered, 8 hours.
Preheat oven to 275°F. Heat a large Dutch oven over medium-low. Add 3 duck legs, skin sides down, to Dutch oven; cook, undisturbed, until deep golden brown while slowly rendering duck fat, about 25 minutes. Transfer legs to a rimmed baking sheet. Pour rendered fat into a heatproof bowl. Repeat process using remaining duck legs, reserving fat. Chill rendered fat in bowl until ready to use.
Return all duck legs to Dutch oven; add 6 cups water, onion, thyme, garlic, and bay leaf. Bring to a boil over medium-high. Cover Dutch oven, and transfer to preheated oven. Roast until duck is just fork-tender and cooked through, 1 hour and 15 minutes to 1 hour and 30 minutes. Remove from oven. Let mixture cool, uncovered, in Dutch oven at room temperature 1 hour.
Transfer duck legs to a rimmed baking sheet; tent with aluminum foil, and set aside. Pour braising liquid in Dutch oven through a fine wire-mesh strainer into a heatproof bowl; discard solids. Skim and discard fat from liquid; return liquid to Dutch oven. Bring to a boil over medium-high. Reduce heat to medium-low; cook until liquid is reduced to about 2 cups, about 40 minutes.
Meanwhile, stir together butter and flour in a small bowl until smooth. Gradually add butter mixture to reduced braising liquid, 1 teaspoon at a time, whisking constantly, until completely melted and incorporated. Cook over medium-low, whisking occasionally, until raw flour taste cooks out, about 3 minutes. Season with salt and pepper to taste. Keep sauce warm over low until ready to serve.
Heat 2 tablespoons reserved duck fat in a large nonstick skillet over medium. Place 3 duck legs, skin sides down, in skillet; cook until skin is golden brown and crisp, 3 to 4 minutes. Flip and cook until duck legs are just heated through, 1 to 2 minutes; transfer to a plate. (Do not wipe skillet clean.) Repeat process using 2 tablespoons duck fat and remaining duck legs. Reserve and refrigerate remaining duck fat for another use.
Arrange spaetzle, mushroom ragout, and duck legs on a platter. Serve with braising sauce.
