Translation:He wants to eat only potatoes for lunch.
Картошка is an instrument of his обед. =) Обедать is not an action of eating, so we don't use Accusative here like кушать картошку. The literal translation would be "he wants to have lunch with/by (?) potatoes" and not "to have lunch potatoes", maybe this way it would be easier for you to remember.
2E3S, thank you very much for your explanation. I would have never understood the use of the instrumental case in this sentence
That means, everything one eats requires instrumental? If I wanted to say e. g.: "I eat onions", it would be: "Я ем луком."?
No, maybe I got it wrong. This is only for the word обедать, isn't it? Not for есть. Maybe also for завтракать and ужинать? --> Я завтракаю яйцом. (?)
Normally, not always, 'just/ only' come after the subject of an English sentence.
При медленном воспроизведении картолька превращается, превращается картолька в элегантную картошку.
"Have potatoes for lunch" marked as incorrect. Обедать - as "have lunch?" Is that not correct? I chose have lunch because the word for "eat" was not present and deliberately left it out.