A restaurant to linger
A restaurant to linger? Interesting. Are we talking dim lighting, Amy Winehouse in the background, a decent bottle of red, perhaps a bit of candlelight? A few flakes of snow falling outside? Stone walls and a planked, wooden floor along with a beamed ceiling? Downstairs at 219 or Al Porto. Forget about the food at both-you are going for ambience and mood and...a long burning candle.
A restaurant to return to...for a future anniversary.
Evening Star in Del Ray (Alexandria) is good. They are decently priced, the foods good and they have a bar in the back, as well as a lounge upstairs. Nice chill ambiance, too.
You might also consider Rustico in Old Town (well, at the very far end of Old Town) and chill there for a bit and hit Buzz, a desert, coffee, drinks place that's is right across the tiny street and stays open until midnight.
I concur that Tallula might be OK, too.
I can't think of anywhere worth noting in Old Town off-hand. Two restaurants that might be good to try ar the Majextic and Eastmans, but I'm not sure if that's quite what you are looking for...
Tallula is always great about letting you linger, plus you can go through the door to EatBar if you want.
Eleventh St. Lounge wil let you linger all you want and it has good food, but it is a lounge and can get a little smokey later in the evening, especially on weekends.
Jaleo in Crystal City has always been good with my group of friends about lingering.
I am not as good with Old Town we don't make it down there often enough anymore.