In A Saucepan, Saute Onion In Butter; Blend In Flour. Add Milk And Cream; Cook And Stir Until Thick And Bubbly. Remove From The Heat; Stir In Salt, Pepper, Pimiento, Water Chestnuts, Lemon Juice, Parsley, Crabmeat, Shrimp, Rice And Half Of The Cheese. Spoon Into A 2-1/2-Qt. Casserole. Bake At 350 Degrees F For 25 Minutes Or Until Heated Through. Sprinkle With Remaining Cheese Just Before Serving.