Onions are a key ingredient in any ceviche, no matter which country is it from. Translated into English it would mean Do you wan the onion? which is the only correct answer. ¿Usted prefiere la cebolla? means do you prefer the onions, ¿Usted tiene la limonada? means Do you have the lemonade? and ¿Usted quiere la limonada? means Do you want the lemonade?
Remember also, in Spanish you use question marks both at the beginning and at the end of a sentence.
