Preheat oven to 350 degrees F. Cook pasta according to package directions; drain. Return to pan.
Meanwhile, remove meat from container, reserving juices. Using two forks, shred meat into bite-size pieces. Set aside.
In a large skillet, cook mushrooms, onion, and garlic in hot butter over medium heat for 4 to 5 minutes or until tender. Stir in flour and tomato paste. Gradually stir in meat juices, broth, Worcestershire sauce, paprika, salt, and pepper. Cook and stir until thickened and bubbly. Remove from heat. Stir in 1/2 cup of the sour cream.
Stir shredded meat and mushroom mixture into cooked pasta. Transfer mixture to an ungreased 3-quart casserole. Bake, covered, about 30 minutes or until heated through.
Meanwhile, in a small bowl, combine the remaining sour cream, horseradish, and snipped or dried dill. Serve with meat mixture. If desired, garnish with dill sprigs. Makes 6 servings.