This question arose in the discussion of a language learning app. Its original sentence is in Russian and it says

Это пюре а вот пицца.

The translation from the app is

This is mashed potatoes and here is a pizza.

Sometimes, the app gives a literal translation instead of proper English. In this case, I'd like to know whether it's correct or not. As a learner, I'd say

• This is a mashed potatoe.

but

• These are mashed potatoes.

I'm thinking that if a bunch of potatoes are mashed (although the shape becomes soft and blended) aren't they originally from some potatoes? Or is it possibly that the app considers "mashed potatoes" as an uncountable noun, therefore it goes with "is"?