sunday, near the Walters
It depends on what you are in the mood for...
Check on Cafe Mocha's hours (they are weird). But their sandwiches are top notch for lunch. IXIA and Sotto Sopra are on the higher end of the price spectrum but are walkable. Mick O'Sheas has some great bar style food, and I think on Sundays they have an awesome sheppard's pie special and is also walkable.
Since you're going on a Sunday, some restaurants in the area will be closed. Here's a quick list of some places:
Egyptian Pizza (always changes names so I'd check the latest - maybe Cafe Isis?)
Mt. Vernon Stable
Helmand (dinner only)