"A bright red mushroom that grows in hot climates. Imbued with the power of heat, they can be used to cook dishes that will allow you endure the bitter cold."
The Sunshroom is a type of mushroom and material found in Breath of the Wild. This mushroom will restore ½ of a heart when eaten raw, but can be increased if Link decides to cook it into a Toasty Sunshroom over an open flame. Sunshrooms can be used in a variety of Recipes, and they have an extra cooking effect of cold resistance. Sunshrooms can also be used to create Red Dye at the Kochi Dye Shop in Hateno Village.
They can typically be found growing in areas of hot climates, such as the Eldin region, where they are fairly abundant in the Trilby Valley. Over 20 Sunshrooms can be found in the area just east of Gut Check Rock, north of Death Mountain. They are also found more sporadically in the Wasteland region as well as the northwest portion of the Lake region.
When upgrading the Snowquill Set, which consists of the Snowquill Tunic, Snowquill Headdress, and Snowquill Trousers, Link will need to use Sunshrooms. The Level 3 upgrades require 5 Sunshrooms, as well as 8 Fire Keese Wings.
After Link has upgraded his Sheikah Slate and gained the Sheikah Sensor ability, this will open up the Sunshroom Sensing sidequest. It is given to Link by Symin, who is the assistant of Purah. It functions as a tutorial, teaching Link how to properly use the sensing ability. A Sunshroom can be found right on the backside of the Hateno Ancient Tech Lab. After taking a picture of it and showing it to Symin, he will ask Link to bring him three Sunshrooms. There are plenty of Sunshrooms near, just northeast of the Hateno Ancient Tech Lab, located in the Retsam Forest. After bringing him three of these mushrooms, he will reward Link with three Hearty Truffles.
Hyrule Compendium entry of the Sunshroom.