You're right. You need to cook every recipe in each part in order to progress in each stage of the game. It just so happened that the special mushroom thing was the last one I made for that areaI may be mistaken, but I thought that you needed to cook all recipes from that stage, and later from 3 other stages. Essentially, complete a cookbook, that's the plot. As far as I got it, was playing in JP and sometimes were skimming text, too lazy to properly read it.