Chicken Stroganoff

Cook mushrooms and onion in 3 Tbsp butter in a 12-inch heavy skillet over medium high heat, stirring occasionally, until mushrooms are browned, 8 to 10 minutes. Transfer mixture to a bowl and reserve skillet for making sauce.

Cook pasta in a pasta pot of boiling salted water until tender. Reserve 1/2 cup cooking water and drain noodles. Toss noodles with 2 Tbsp butter in a bowl.

While pasta is cooking melt remaining 2 Tbsp butter in skillet over medium low heat, then stir in flour and cook roux, stirring up brown bits, 3 minutes. Stir in tomato paste until well combined, then add stock and bring to a boil, stirring. Add mushroom mixture and simmer 2 minutes, then gently stir in chicken, scallions, sour cream, 1/2 cup pasta cooking water and salt and pepper to taste. (Do not let boil.) Serve chicken over noodles.