This was the best explanation I could find: https://www.reddit.com/r/Korean/comments/hlqxwd/%EB%A7%9B%EC%A7%91_vs_%EC%8B%9D%EB%8B%B9_vs_%EC%9D%8C%EC%8B%9D%EC%A0%90/.
It's not completely natural, at least not to my native English (US) ears. It's hard to explain why but I'll give it a shot.
In English you use "where is/are" for specific things (usually+, things that the listener is expected to know which thing(s) you're looking for, often because they have been mentioned previously in the same conversation or in a prior conversation), but for such things we use the definite article (the) if needed++, not the indefinite article (a/an).
If you want to ask where something is without specifying which one, you can ask, "Where can I find a restaurant" instead of "where is".
+In addition to referring to something previously mentioned, you can also ask where is/are or use the definite article if you add information to specify which specific thing--for example, "Where is the key to this door?" works because "to this door" specifies what key you're talking about. Additionally, if that information can be inferred, then you don't even need to add it. So if I can't just ask "where's the key" if there's no way the listener would know which key I'm looking for, but if someone can see that I'm trying to open a door, then they will know which key I mean. Finally, you can use "the" and "where is" if the item is logically specific even if neither you nor the listener know which item it is. For example, I could ask "Where is the smartest man in the world?" and, even though neither I nor the listener knows who that is. That's a silly example because you can't answer if you don't know who it is, but it's grammatically correct. A better example would be, "Where is the highest mountain" because it is logically specific and the listener could look it up even if he/she doesn't know which mountain that is.)
++Generally, place names (usually proper nouns but not always) don't need "the" unless it is part of the name, and objects/things do. So "hill" needs "the" but "Bunker Hill" does not. But rivers are an exception--we say "the Tiber River" or even "the Tiber" but we need to use "the" for rivers (possibly because we think of rivers as things instead of locations). Another point of confusion for some is heaven/sky; "sky" needs "the" but not "heaven" (which we think of as a place name), and usually "home" doesn't need "the" when no additional information is needed to specify whose home. All this is really difficult to lay down hard and fast rules for.
A more natural parallel would be "Where is there a restaurant?".
When you ask for the location of something, there is a possibility of the thing not existing. In English, the speaker can observe this possibility by using the indefinite article and changing the question from "Where is the restaurant?" to "Where is there a restaruant?"
In the former, the speaker is assuming that the restaurant in context exists. In the latter, the speaker is unsure or ambivalent about which restaurant.
To my ear, that 은 on the end of 음식점은 makes it "the restaurant." Topicalizing "restaurant" suggests we know there is one around here. In English, if I'm hoping for one, "Where is a restaurant?" is less likely than "Where is there a restaurant?" but most likely I would say "Is there a restaurant around here?"