Well, the brain-washed minion thing would be my first choice for a quick snack. I also refined the sweet pods into sugar for eating during the last turn, so we are not completely out yet. But yes, we need a more reliable and efficient food supply. And with "efficient", I mean "least amount of total shifts spent necessary to feed everyone".
Farming is the one option that immediately springs to mind for those who can digest plants. Since it's pretty boring, I suggest making the imp do it. Tilling soil and putting seeds into the ground should be simple enough that even it can manage. Optimally, the Sprite can cast its fast growth magic on the fields and otherwise concentrate on more challenging tasks.
As for our carnivores: this is trickier. The optimal solution would be a herd of grazing animals, upgraded by our resident mad scientists so that they regenerate lost limbs. Put them in a pasture and forget about them until it's time to sever a limb for eating or to suck some blood. Since we don't have any grazing animals, we should probably launch an expedition to the surface or to that new cave behind the sludge warrens to catch some. We'll need to find a way to safely deal with the slime first, though. (Trap it in a container or poison it with a molotov or acid potion, perhaps?)
In the meantime, fishing, hunting, and vermin collection are an inefficient stopgap measure. Maybe the time efficiency of hunting could be improved by setting traps and collecting what they gather from time to time?
Finally, there is the issue of brain lobes for the illithid, but I think a project for growing sentient plants is already well underway.