All characters in Genshin Impact have a special dish that only they can cook. Getting it can be a little awkward, as you need the character to cook another meal, and you then have a random chance of obtaining the special dish from it.
For Thoma, that special dish is Warmth, hearty miso made of seagrass and tofu. Thoma will have a chance of making Warmth when he is used to cook the Miso Soup meal. Miso Soup restores 8/9/10% of Max HP to the selected character and regenerates 210/260/300 HP every 5 seconds for 30 seconds.
Tofu can be purchased from vendors in Liyue and Inazuma. Aoi in Inazuma City will sell it for 100 Mora, as will Dongsheng in Liyue Harbor, Ms. Bai in Qinqce Village in Liyue, and Kiyoko at Bourou Village in Inazuma will also sell tofu to you for 100 Mora.
Seagrass can be found in the shallow sea at beaches all across Inazuma and is very easy to find and harvest. When they are combined in a cooking pot by Thoma to make Miso Soup, they will have a chance of making Warmth instead.
Warmth restores 14% of Max HP to the selected character and regenerates 350 HP every 5s for 30s, acting as a buffed version of the Miso Soup.