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Cook pasta according to package directions; set aside.
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Melt 1 tablespoon butter in a large skillet over medium-high heat; add chicken (sprinkle with salt and pepper to taste), and cook 2 minutes on each side or until lightly browned.
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Stir in sherry and 1/2 cup water; cover, reduce heat to low, and simmer 20 minutes or until done.
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Drain, reserving liquid; cut chicken into 1-inch pieces; set aside.
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Melt remaining butter in skillet; saute onion and mushrooms over medium-high heat 5 minutes or until tender.
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Stir together flour and 1/2 cup broth.
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Add reserved liquid, flour mixture, and remaining 1 1/2 cups broth to skillet.
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Cook, stirring often, over medium-high heat until thickened.
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Remove from heat; stir in pasta, chicken, sour cream, Monterey Jack cheese, 1/4 cup Parmesan cheese, salt, and pepper.
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Spoon mixture into a lightly greased 13x9 inch baking dish.
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Sprinkle with remaining 1/4 cup Parmesan cheese.
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Bake at 350 degrees for 30 minutes or until thoroughly heated.