With the updated software, I was really cooking with gas. I got the project done in half the time.
Grammatically, a use of the "-ing" form of cook + the prepositional phrase "with gas". Together with a form of be this forms a progressive verb predicate. Only rarely are other forms used in this idiomatic sense.
From the suggestion, heavily advertised in the 1980's and early 1990's, that stoves using natural gas as a fuel cook more effectively than, for instance, wood-burning or electric stoves.
English Wiktionary. Available under CC-BY-SA license.