To unlock this sidequest, you first have to complete the sidequest "Koko's Kitchen". Learn about it here.
Talk to Koko once again between noon and 7:00 PM at the cooking pot near the general store in Kakariko Village. If it's raining you will have to wait out the rain or go pass time near a fire somewhere else in Hyrule.
This time, Koko needs Raw Meat. You can find raw meat by hunting various animals around Hyrule.
Give her some and she will share Tough Meat-Stuffed Pumpkin with you as a reward. In your Adventure Log, the recipe for the dish will be recorded. Cook pumpkin and add any kind of meats to create the dish.
The next sidequest, and the last sidequest in this chain, is Koko's Specialty.