I think that the phrase, "today I am eating only side dishes," would work as well.
Often -고 있다 is used in Korean for the present continuous. I agree though that in certain contexts, they are interchangeable.
"today I am eating only side dishes." : 오늘 저는 반찬만 먹고 있어요. isn't it weird? i think i don't say 오늘 when I am using the present continuous. Many use 지금(Now).
Is there a reason why there's not object marker on "only side dishes"? Can that be clarified?