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1
Heat butter and oil in a deep 10-inch heavy skillet over medium heat until foam subsides.
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2
Meanwhile, pat duck dry and sprinkle with 1/4 teaspoon each of salt and pepper.
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3
Sear duck, skin side down, until golden brown and some of fat has rendered, about 6 minutes. Turn over and cook until browned, about 2 minutes more. Transfer duck to a plate, then add onion to fat in skillet with garlic, rosemary, and 1/8 teaspoon salt and cook, stirring occasionally, until onion is softened and golden brown, about 6 minutes. Add wine and boil 3 minutes.
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4
Return duck, skin side up, to skillet, then add any juices from plate, stock, and tomatoes with their juice. Bring to a boil, then gently simmer, covered, 1 hour.
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5
Transfer duck to a cutting board, then skim off about three fourths of fat from sauce and discard.
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6
Puree sauce in batches in a blender (use caution when blending hot liquids). Return sauce to skillet and boil, stirring occasionally, until reduced to about 2 1/2 cups, about 8 minutes.
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7
While sauce reduces, finely chop duck with skin.
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8
Return chopped duck to sauce and season with salt and pepper.
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9
Cook tagliatelle in a pasta pot of boiling salted water (3 tablespoons salt for 6 quarts water) until al dente, then drain pasta and toss with duck
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10
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