"He cooks an onion."
Translation:Cucina una cipolla.
"cook" translates both "cuocere" and "cucinare", but in this case the right option is the second one. It means he is preparing an onion as a meal, but maybe he is still in the "chopping up" phase. You can't say for sure he's applying heat ("cook" with the meaning of "cuocere") to the onion, which would be clear if the cooking time was specified.
@ItalianaAmore Both "egli" and "lui" are correct, though the former is mainly used in written (specifically literary) language. If the latter is not accepted, report it.
@JohxgU Because "cucino" means "I cook" (first-person singular), whereas "cucina" is a third-person singular (he/she cooks).