Is сварить really only limited to cooking via boiling? "We need to cook potatoes and onions" was marked wrong for not using "boiled."
I ask this because, when searching the Internet for actual Russian examples of сварить in literature, I come across many examples like:
кто ему сорочку выстирает, кто ему есть сварит?
Who will wash his shirt, who will [boil food (?)] for him?
It seems to me that the Russian author intended сварит to mean "cook" in a more general sense -- as general as, for example, the verb готовить. Am I wrong?
It's complicated. "Варить" is to cook by boiling. In the context of Duo's sentence it can't mean anything else. But boiled food, especially soups are such an essential part of Russian cuisine that the word "варить" can be used in general statements like the one you've found. Soups are considered to be universal meal to the point where Russian equivalent of "stay in kitchen" is "варить борщ".
I'm really enjoying Duolingo Russian and the Tips are indispensable. In the Food Skill, the Tips could use a rewrite.
For example, the concept of mass nouns (like "лук") is introduced with this statement. "Food offers a delicious intake of mass nouns. Russian has them massed up even where English does not!" It's likely the writer was trying to be clever, but these sentences are poorly written and the meaning is unclear. The tips should be simply put and easy to understand.
It would be more useful if they defined "mass noun" and then went on to say that the Russian language has many mass nouns that are related to food, then point out that there are mass nouns in Russian that are not mass nouns in English and include specific examples.