"The big onion."
Translation:An t-uinnean mòr.
Because the article an in nominative prefixes t- to masculine nouns beginning in a vowel, and uinnean is a masculine noun beginning in a vowel.
It is explained in the tips and notes to the Food 2 skill. The tips and notes are available in the web browser version of Duolingo (go to the https://duolingo.com website) – unfortunately they aren’t present in the mobile app. You can also read them on the duome.eu website.