Pre-heat oven to 400 degrees F. Wipe mushroom caps with a damp cloth to remove any dirt and remove stems. (Discard stems or save for another use.) Set aside caps. In a saucepan on low heat combine and stir with a wire whisk: flour, Neufchatel cheese, milk, garlic, salt, nutmeg, pepper and heat gently until cream cheese is no longer lumpy. Stir in mayonnaise, lemon juice, Parmesan cheese and green onion. Remove from heat and fold in crab meat and shrimp gently with a rubber spatula. Cover and allow to cool completely before stuffing the mushrooms. Stuff each mushroom cap with filling. Place in a 9 x 13 inch greased baking dish and sprinkle with cheddar cheese and seasoned bread crumbs if desired. Bake 15 to 20 minutes.