Preheat oven to 400 degrees F (200 degrees C). In a medium-sized mixing bowl, stir together mayonnaise, sour cream, Parmesan cheese and onion. When these ingredients are combined, mix in artichoke hearts, lemon juice, salt and pepper. Transfer mixture to a shallow baking dish. Bake at 400 degrees F (200 degrees C) for 20 minutes, or until light brown on top.