Probably an insect in the grub stage, bred to grow very large, would fit the bill. Already, the palm beetle grub is a delicacy in the tropical places where it's found. Wikipedia only lists Asia and Africa, but it's also popular in the Amazon.
If you do an image search for palm weevil larvae, you will find images that mostly depict them cooked and prepared for eating. Not only are they nutritious, they are also tasty:
Sago grubs have been described as creamy tasting when raw, and like bacon or meat when cooked.
The larval form of insects is good to use as a food source, since biologically, their drive is to store up fat to burn later when they metamorphose into their adult form. If you harvest them in adult form, they've already used up that energy in metamorphosis.
Palm Weevil grubs would be a good candidate, because all they need for grazing is basically sawdust, palm wood stock, etc.