To unlock the Collector achievement, you need to complete all of Susan's collection. Susan's food collection is different from the kitchen collection. To fill Susan's collection, you will need to cook every single food item for it to register.
For PlayStation players:
Due to its status as a Platinum, you need to unlock all other trophies/achievements in the game before you can receive Susan’s Collector.
Since platinums are always “unlock every other trophies”, we figured we’d spice it up by making it on theme… but we failed to realize just how confusing it would end up being for the players in the end. Sorry for the confusion this has caused!
Congratulations on completing the collection though!