Minecraft wiki entry for Quick_ways_to_get_food, associated text: Mushroom stew is still a good option: Both kinds of mushrooms are common, and bowls can be made from giant fungus planks (the red and blue "trees"). Alas, flowers are not to be found (unless you brought some from the Overworld).
Rotten flesh (see above) can be had from zombified piglins; the problem is that if you fight one of them, you basically have to fight all of them. You can do pretty well with a prepared fort where you can hold them off and kill them from safety. You can try fighting them from atop a two-block pillar, but but watch out for other monsters (especially ghasts or skeletons) who can knock you off your pillar, or blazes which can set you afire.