There's mention in the film and film script that he's cooking something called rootleaf.
CREATURE: Not far. Yoda not far. Patience. Soon you will be with him. (tasting food from the pot) Rootleaf, I cook. Why wish
you become Jedi? Hm?
The original junior novelisation describes it as "rootleaf" soup
“Not far,” said the creature. “Yoda not far. Patience. Soon you will
be with him.” He tasted the food directly from the pot. “Rootleaf, I
cook. Why wish you become Jedi? Hm?”
This concurs with the new (canon) junior novelisation, albeit it's now "root-leaf stew". There's also some cooking advice. Evidently it also contains swamp weed.
Once in the house, the creature immediately busies himself with
cooking. He has a pot hung over a little fire, and two small cauldrons
sitting among coals, and he runs back and forth between them, throwing
roots and swamp weed into them. As he cooks, the smell that wafts
through the room reminds you of a garbage depot in the hot season on
The creature shoves a spoonful of root-leaf stew into your mouth. It
has the consistency of boiled brains, and you’re about to spit it out
when you stop—it doesn’t taste terrible, actually. You chew and
In the Star Wars Encyclopedia, it's described as a nourishing stew
Rootleaf: A type of plant native to Dagobah. The Jedi Master Yoda used rootleaf to brew a potent and nourishing stew.
In the factbook "Star Wars: Absolutely Everything You Need to Know" it's back to being rootleaf (with no hyphen) and it's a soup again.