Zelda Tears of the Kingdom: How to complete A New Signature Food Side Adventure
One of the more amusing Side Adventure chains in The Legend of Zelda: Tears of the Kingdom would be the mayoral election of Hateno Village, between the incumbent Reede and the challenger Cece - and both candidates will enlist your help to try and garner an edge for their chances with the voters. For Reede's request, he wants you to help rediscover what once had the chance to be a signature food for Hateno Village; a recipe that his grandfather had worked on with another villager, but has since been lost to time. Thankfully, he's not the only villager looking to revisit this once-forgotten recipe - and you'll want to discuss the situation with Koyin at the village's Dairy Farm, which will unlock her own connected Side Adventure, A Letter to Koyin.
How to complete A Letter to Koyin
This one is simple; if you talk with Koyin at the pier to the lake behind her family's farm, she'll note that she was researching a forgotten family recipe when the bottle containing the instructions fell out of her hands, and rolled its way deep into the lake. Thankfully, the bottle was corked and is safe - but she has no recourse to fish it out of the water, which leaves things up to us.
Unfortunately, the range of our Ultrahand won't be enough to simply scoop the bottle out of the brink alone - so it's time to make ourselves a makeshift dock to better reach the bottle. Simply chop down a few trees along the shore, stick them together, and then position it towards the portion of the lake the bottle has sunk to. About 4 logs should be enough here to get enough reach to grab the bottle. Once you can reach it, simply drag it out of the water with Ultrahand, and bring it to Koyin. She'll quickly set to work researching the notes, and discover that her grandpa's old recipe was for Hateno Cheese - which apparently hadn't gone over well when he and his business partner first attempted to introduce it, but seems likely to be a hit with a more modern crowd. Not only will the cheese now be available at the town's general store, but if you bring fresh milk to her she'll gladly turn it into cheese free of charge.
How to complete A New Signature Food
If you've completed A Letter to Koyin as instructed above, Reede's own request will be complete upon reporting to him. A little odd that there are two Side Adventures for essentially the same quest, but either way - congrats on the cheese, and have fun experimenting with using it in your cooking recipes!