I.e., can man live on bread alone?
And water and air of course. And a Centrum Daily Multivitamin.
I’ve heard dogs can live on just corn and soy meal.
And people get all the key proteins from rice and beans.
But is there one food that would do it?
I’d allow mixtures like bread, meatloaf, or chicken noodle soup.
And no, I don’t want ad hoc combinations like “Just put your daily diet in a blender”.
Nor any SciFi refernces to the daily food pill.
Real food that already exists.
“Meat loaf” and “chicken noodle soup” are not “one food”. So why not a stew of any grain with any legume, with a dollop of V8 sloshed in, and a slice of cheese melted on top?
I asked a version of this myself a long time ago but it was more restrictive. I asked about a single ingrediant that would do the trick )e.g. brocolli, beef, or humpback whale blubber). I don’t think anyone came up with a longterm solution. Given your criteria, you could easilly make pizza that could sustain you forever but that isn’t an interesting answer UMHO because you can have all the food groups accounted for on one pizza. Scurvy is a big threat to this type of idea so you have to acoount for enough Vitamin C.
Or, if you pick something that’s pretty nutritionally complete, like eggs, it has zero fiber and therefore isn’t feasible for humans to live on long-term.
Actually, that is a complete answer. Cecil only poses water and vitamins as problems, but the OP allows those.
Does the goop that goes into feeding tubes count?
Pizza
potatoes 9with the skins and a little milk and butter). it sustained the Irish from ca. 1600-1840. of course, if you can’t grow them you have a problem.
Beer, for a broad enough definition of “beer.” If I’m allowed to vary the beers that I drink, I can survive indefinately on beer.
Extra lemon in your hefe-weizen, sir? You’re looking a little…well, scurvy. 
I was thinkung CaveCreek Chili Beer, but hefe will do quite nicely. 
Haggis. I’d rather die than eat it again. End of problem.
Added benefit to hefe is that it has a good bit of fiber…always hard to come by on an…ahem…liquid diet.
Wouldn’t you die of alcohol poisoning at some point?
Drinking beer? Not likely, assuming you’re just drinking steadily throughout the day, and giving your liver time to do its thing.
I was gonna ask about this. My dad claimed that that’s all he ever saw his parents eat when he was young.