Since the onion joke is probably incomprehensible to someone working only from the translation:
The shopping list, written in hiragana, calls for one negi ねぎ (bunching onion, the green leek/scallion things). But the negi at the supermarket are labelled with the word in kanji (葱), which Katyusha can't read. The onions (tamanegi, literally "ball negi") have the word written with the "negi" part in hiragana (玉ねぎ), so they match the list.