。
I want more steak.
Literal
Steak [subject-が] more want.
Subject is omitted (the speaker). 欲しい ('to want [a thing]') is an i-adjective that takes が for its object — 'the thing wanted' is grammatically the subject. もっと ('more') is an adverb, not quantifying anything specific. Distinct from ~たい ('want to do'), which attaches to verbs.