In a large saucepan, sauté mushrooms, onion and garlic in 3 tablespoons butter until tender. Stir in flour until blended. Gradually add milk. Bring to a boil; cook and stir for 2 minutes or until thickened. Remove from the heat; stir in mayonnaise. Add the chicken, rice, celery, peas, pimientos, lemon juice, salt and pepper; mix well.
Transfer to an ungreased 13-by-9-by-2-inch baking dish. Melt remaining butter; toss with cornflake crumbs. Sprinkle over top. Bake, uncovered, at 350 degrees for 30-35 minutes or until bubbly.
*Editor's Note: Reduced-fat or fat-free mayonnaise may not be substituted for regular mayonnaise.
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