Once you're done Demeter, you can return to talk to her and she will tell you that she has a big problem and she will only tell you about it once you are a Level 5 Chef. When you reach Level 5 talk to Dem again and she'll tell you that her daughter is missing. There are a few different places she can be; EL has most/all of them. Talk to Per and she'll give you a fruit to take back to Demeter so she knows she is safe. Bring it back and Dem will give you the Persephone's Cornucopia recipe.