Guinea pigs.
Seriously. People in the Andes eat them. They gain well on only a little bit of feed, they can stand cold weather, they pack on quite the little bit of meat.
Since urbanites don’t have feed growing naturally (due to the lack of land), they have to bring in all their feed. They should be most concerned about what farmers call “conversion efficiency” - ie, how many pounds of meat do you get per pound of feed in. Sheep have a higher conversion than cows, for example, and goats a tad higher than sheep, and chickens higher yet. You get the idea.
But sheep taste so much like mutton...
Yep, I get the idea.
But pigs can be fed garbage. So can rats. can guinea pigs?
BTW, the most efficient meat production is ostriches, if I remember correctly. But I doubt a city dweller can raise one by feeding it city dweller garbage.
In Colombia (SA) the people feed their guinea pigs scraps from the kitchen. When they’re big enough it’s dinner time.