In a saucepan over medium heat, saute mushrooms, onion and garlic in 3 tablespoons butter until tender. Stir in flour until thoroughly combined Gradually add milk; bring to a boil. Cook and stir for 2 minutes or until thickened and bubbly. Remove from heat; stir in mayonnaise until smooth. Add chicken, rice, celery, peas, pimientos, lemon juice, salt and pepper; mixwell. Spoon into an ungreased 12x9x2=inch baking dish. Melt remaining butter; toss with cornflakes. Sprinkle over casserole. Bake, uncovered, at 350 for 30-35 minutes or until bubbly.