Translation:Where is the restaurant?
In English this seems pretty natural, like the kind of thing that you'd say if someone was talking at length about a particular restaurant but hadn't yet told you where it was. Also, I could imagine saying it in a situation where we were going to meet people at a restaurant in a particular area, like a shopping plaza or a particular small town or commercial center, but we hadn't found it yet, and then we ask "Where is the restaurant?"
The answer is "Where is a restaurant?" Probably because Japanese doesnt directly correlate to English the way we want it.
Whats important to understand here is that this specific sentence is asking for 'A' restaurant. Similar to how へやです。Just means, "It is a room." Or similar.
LearnJapanesePod has a good lesson on Ski, the word for Like. (I like food. Etc.) I suggest checking it out.
The answer is not 'Where is a restaurant'. どこ is asking for a specific location, not a generalization. Also, 'a' means 'one'. This is strange in English as it is in Japanese. One wouldn't say 'Where is one restaurant'.
どのへん・どのあたり would be closer to asking 'Where are some restaurants', but it could also be referring to the general location of a specific restaurant that is being inquired about.
I have used and heard this expression from Japanese native speakers to inquire about the location of a certain restaurant that we are supposed to meet at. It is a completely standard inquiry to 'Where is the restaurant?'.