dish

I don't really get the difference between "dish" and "food".
What should I say : "Oysters is the most disgusting food I've ever eaten" or "Oysters are the most disgusting dish I've ever eaten?" And should I say "are" or "is". Is the verb supposed to agree with "oysters" or "food"?
Hope I made myself clear...

I'd use "food" rather than "dish" in this sentence. I'd also use "are".

I don't know if this will help much but here goes.

Based on the dictionary definition of the word "dish" either of your examples is correct. However, I don't think that a native speaker (which I am, by the way) would ever use the word dish in an example like this. Were I to say something like this I would say "Oysters are the most disgusting food I've ever eaten".

I don't claim to be a grammarian and I have to admit the agreement here has me confused as well. Not because I don't know the proper way to say this but because I can't explain why on the one hand "oysters are" which is clearly plural yet on the other hand "food" is singular. I'm going to guess that for purposes of agreement with "food" "oysters" is simply construed as singular. I'm not sure that's much of an answer but it's all I've got.

As to when to use "dish"--typically, I guess, dish is used to refer to a non-specific preparation of food as "the cook outdid himself with that dish he made for dinner" or "everyone is going to bring a dish to work for the Friday lunch".

Here's an example I found on a food related blog: "You don’t see any upscale restaurants with pictures on their menu, just a detailed description of the dish."

Hope this helps. Perhaps others can be clearer or more specific.

In my experience as a native english speaker (in North America) dish would not be commonly used to describe food itself. More commonly about the object it is served on/in. Plate, Bowl, etc. Or as it is presented. Like: "The waiter served the customer a dish of Oysters." Although even then the most often word dish wouldn't be included. Seems to me it's an extra word which wouldn't add anything to the sentence.

For example, asking someone to go clean the dinner plates and other objects used utensils, cups/glasses etc. would be commonly stated as "Please go do/wash the dishes."

Overall, It depends on the sentence structure around it and what it is describing and the region.

"food" is a mass noun, so there is verb agreement in the sentence "Oysters are the most disgusting food I've ever eaten."

<<In my experience as a native english speaker (in North America) dish would not be commonly used to describe food itself.>>

No, not always. We're talking about a whole different meaning og the word "dish" here. "Dish" in the sense of food usually refers not to a single item like "oysters", but to a more complicated preparation and presentation involving more than one food item. Thus, potatoes are a food, but potatoes au gratin is a dish, since now the potatoes have been sliced and cooked and mixed with other ingredients.