A roast (countable), I would say, from the point of view of the shopping list and the cook. Sometimes called (a) pot roast.
From the point of view of the family eating Sunday lunch, also just roast beef (noncount).
See if others agree. Nowadays people probably make them in an Instant Pot or something, but I think the original idea was that the roast would be cooking slowly during the morning while everyone was at church.
A search in the BYU text corpus for AE on 'roast' plus the collocate 'Sunday' still brings a fair number of hits.
[A]h, the sunday roast [.]For many Americans, having a roast for Sunday dinner is as certain as the day itself.
On Sunday there was a roast, Monday you had cold slices because that was the day everyone did laundry
(memories of British wartime cooking, in a US newspaper; 'slices' is probably BE)
So pack up the last of Sunday's roast beef for sandwiches