How to Make It
Using a sharp knife or poultry shears, cut off last 2 joints of duck wings; set aside for broth. Trim and discard excess skin around neck cavity. Remove neck and giblets from cavity; reserve for broth. Reach into cavity; pull away any fat deposits, and discard. Using a metal or wooden skewer, prick skin of duck all over at a 45 degree angle, taking care not to pierce the meat.
Stir together 2 tablespoons brown sugar, 2 tablespoons salt, and juniper berries in a small bowl. Sprinkle 1 tablespoon brown sugar mixture inside cavity of duck, and rub remaining mixture over skin. Place duck on a nonreactive wire rack set inside a rimmed baking sheet, and chill, uncovered, 18 to 24 hours.
While duck chills, preheat oven to 375°F. Chop reserved wing tips and neck into 2-inch pieces. Place on a rimmed baking sheet with onion, carrot, and celery; add oil and 1/4 teaspoon salt, and toss to coat. Spread in a single layer, and roast in preheated oven, stirring occasionally, until well browned, about 55 minutes. Add 1 cup cold water to baking sheet, stirring and scraping up browned bits from bottom of pan. Transfer mixture to a large saucepan; add reserved heart and gizzards. (Save liver for another use.) Stir in remaining 3 cups cold water and 2 thyme sprigs from 1 bunch. Bring mixture just to a simmer over medium-low. Partially cover, and simmer very gently (do not boil) until reduced to 11/2 cups, 1 hour and 30 minutes to 2 hours, skimming and discarding any foam that accumulates on surface. Pour mixture through a fine wire-mesh strainer over a bowl; discard solids. Chill broth until ready to use.
Preheat oven to 250°F with oven racks in lowest and middle positions. Let duck stand at room temperature 30 minutes. Pour water to a depth of 3/4 inch in a large baking pan, and place on lowest oven rack. Transfer duck, breast side up, to a roasting rack set inside a deep, aluminum foil–lined roasting pan; insert 2 bunches of thyme into main cavity. Prick skin all over once again with a skewer to ensure rendering. Place roasting pan on middle oven rack, and roast at 250°F until a meat thermometer inserted in thigh and thickest part of breast registers 145°F, about 1 hour and 30 minutes. (Breast may take longer to register 145°F.) Remove roasting pan; drain and discard cavity juices and drippings in pan. Remove water pan from oven, and increase oven temperature to 450°F.
Once oven has preheated to 450°F, return duck in drained roasting pan to middle oven rack. Roast until skin is dark and crisp, about 20 minutes. Meanwhile, skim fat off surface of chilled duck broth, and discard. Transfer skimmed broth to a small saucepan, and bring to a simmer over medium. Stir in remaining 1 teaspoon brown sugar. Cook until reduced to 1/3 cup, about 20 minutes. Stir in remaining 1/4 teaspoon salt; set aside, and keep warm.
Remove roasting rack with duck from roasting pan. Drain and discard foil and drippings from pan; wipe pan clean. Return roasting rack with duck to roasting pan; set on a heatproof surface, and let rest 15 minutes. Remove and discard thyme in cavity. Quickly run remaining 2 bunches thyme under water to dampen; stuff damp thyme into cavity.
Place kirsch in a heatproof measuring cup with a spout. Heat a small saucepan over medium until warm, about 30 seconds. Remove saucepan from heat, turn off burner, and pour in kirsch. Using a utility lighter, carefully ignite fumes just above surface of kirsch. Slowly and carefully pour flaming liquid over duck. Once flames extinguish, transfer duck to a cutting board. Pour residual kirsch from roasting pan into reduced broth. Add frozen cherries, and cook over medium, stirring occasionally, until cherries are heated through, about 3 minutes. Remove from heat. Swirl in butter until emulsified. Carve duck, and serve with sauce.