This English sentence does not sound very natural to me.
I believe they are trying to demonstrate that arrosto can be used as both a noun and an adjective. I have heard people say "I have a roast" meaning "I have a piece of roasted meat."
I think it's supposed to be something along the lines of "I have the roast in the oven."
In British English, we'd say this to mean e.g. a Sunday roast.
Same or similar meaning in American English. I translated it as "I am having a roast". This would mean that one is having a get together where a large roasted meat is involved.