Newbie here, so I will try assiduously to adhere to DonRockwell.com style, protocol, culture, & c.
I work on the Senate side right across the street from the Hart Building, and would second many of the recommendations (as well as the tales of the desolation that is the Capitol Hill dining scene) -- particularly that of my good friend LoganCircle in re: East Street Cafe in Union Station (one thing to keep in mind -- they give you absolute loads of food, so either come with an appetite or make sure you ask for your noodles or tofu to go).
An option I choose at least once a week is the cafeteria in the basement of the Dirksen Senate Office Building (the middle Senate building -- between Russell, which is the oldest, and Hart, which is the modern-looking one). The food is cheap (it must be subsidized by the gub'mint -- for example, you can get rotisserie chicken breast and wing, one side dish, slaw and a piece of sweet Northern-style cornbread for $4.35) and the rotisserie chicken is quite good. Oh, and -- when I'm feeling naughty -- the mac-and-cheese is a spectacularly decadent way to spend 600 calories or so. Speaking as a native Southerner, this macaroni would do any Dixie aficionado or other soul food fan quite proud.
I, like others banished to this part of the world during the business day, am quite excited about the advent of Johnny's Half-Shell on the Hill. Oh, and I find the pub-grub staples at the Dubliner perfectly acceptable for a hearty lunch or happy-hour meal (love me some Guinness Burger). And their patio is a great place on a lovely day.