Saute mushrooms in 2 tablespoons margarine in a skillet 5 minutes. Add sherry; cook 2 minutes or until reduced to 1/4 cup. Add 1/2 teaspoon salt and 1/8 teaspoon pepper.
Place chicken in a saucepan; add water to cover. Bring to a boil; cover, reduce heat, and simmer 20 minutes. Drain; let cool, and shred. Cook onion in 1/4 cup plus 2 tablespoons margarine in a saucepan over low heat 15 minutes. Add garlic; cook 1 minute. Add flour, stirring until smooth. Cook 1 minute, stirring constantly. Add half-and-half and chicken broth; cook over medium heat, stirring constantly, until thickened. Remove from heat; add fontina cheese, 1/2 cup Parmesan cheese, 1/2 teaspoon salt, and 1/8 teaspoon pepper. Stir until cheeses melt. Cook linguine according to package directions; drain. Combine mushroom mixture, chicken, cheese mixture, 1/2 teaspoon salt, and 1/4 teaspoon pepper. Stir in linguine. Spoon into a greased 13- x 9- x 2-inch dish.
Combine breadcrumbs and remaining 1/4 cup Parmesan cheese and 1 tablespoon margarine; sprinkle over chicken. Bake, uncovered, at 350° for 20 minutes.