Learn Italian: Quello che nessuno vuole mangiare sono gli avanzi di ieri. - What nobody wants to eat are yesterday's leftovers.

In Italian, cosa (or che cosa) is used for direct or indirect questions. When "what" translates to "that which" or "the thing that" to set up a statement's topic, you must use quello che or ciò che. This creates a pseudo-cleft sentence, which builds anticipation and shifts the emphasis onto the end of the sentence.

Why is the verb sono plural when the first part of the sentence (quello che) seems singular?

In Italian pseudo-cleft sentences, the copula (the verb essere) agrees with the specific noun it introduces, not with the introductory quello che. Since gli avanzi (the leftovers) is plural, the verb must be the plural sono. English speakers often trip up here because we usually default to the singular "is" in this structure.

Could I use ciò che instead of quello che in this sentence?

Yes, ciò che nessuno vuole mangiare is perfectly correct and means the exact same thing. Ciò che is slightly more formal and often preferred in writing, whereas quello che is the standard choice for everyday spoken Italian.

Why isn't there a non in this sentence if it contains a negative word like nessuno?