The Old Man's Diary
The Old Man's Diary is an item from Breath of the Wild, found in the Woodcutter's House, located on the Great Plateau. It is the diary of the Old Man, and as Link progresses in The Isolated Plateau main quest story, more will be written within the diary.
When Link first finds the Woodcutter's House, he can read the Old Man's Diary and it only consists of the first three segments. The Old Man speaks of the Spicy Meat and Seafood Fry recipe, but he has unfortunately forgotten the final ingredient. He remembers that it includes Raw Meat and Spicy Peppers, but does not recall the last ingredient. As its name suggests, the final ingredient is for the seafood, which comes from a Hyrule Bass.
After Link has cooked the recipe, he may show it to the Old Man and will be rewarded with the Warm Doublet. After doing so, the Old Man will expand his diary, adding three more paragraphs, where he speaks more about the recipe, indicating that the warming effecting has a time limit.
After Link has completed The Isolated Plateau main quest story, the Old Man will complete his diary, adding a final portion about the true identity of the Old Man. He apologizes for concealing his true identity as King Rhoam, and asks that Link save his daughter, Princess Zelda.
On this desolate plateau, the only pleasure that brings me comfort is cooking. And today, I outdid myself! Truly, I created the perfect dish. I call it... spicy meat and seafood fry. This recipe not only restores health, but it also keeps me warm, even when traveling in the snowy mountains. With this dish on my side, I no longer have need of that itchy warm doublet.
I do not know how I allowed this to happen, but it seems I forgot to write down a very important recipe. I know it contained raw meat and spicy peppers. However... I simply cannot remember what else I used! My age is catching up to me.
Sadly, on this lonely plateau, I have only my own knowledge and memory to rely on. Still... If I did find someone who knew the missing ingredient, I would happily reward them with my warm doublet. However, it seems unlikely that such a miraculous wish will ever be fulfilled.
To my surprise and delight, I did happen upon someone who was able to re-create my spicy meat and seafood fry. I gave him my warm doublet, and I was happy to do so. He is a fine young man with great potential, that one.
Ah, and I will now write down this perfect recipe so that I will never forget it. Place some raw meat, spicy pepper, and hyrule bass in a pot, and then turn up the heat. Voilà!
Eating this dish allows you to walk the snowy mountains with ease, though I did forget there's a time limit on the effect. It is too bad I already gave away that warm doublet, which keeps you warm no matter how long you wear it.
Link... Bit by bit, you may come to realize who I am. I am sorry for not revealing my true identity to you sooner. The truth is, after you awoke from your long Slumber of Restoration, I did not know how to tell you all there was to say.
Perhaps deceiving you was not the right thing to do. Still...you must admit I put on a great performance! But all joking aside...what I ask of you is of the utmost importance, dear hero. I implore you, with all my heart... defeat Calamity Ganon and save my precious daughter, Zelda. I understand this is no simple task I am asking of you, but I know beyond a shadow of a doubt that you can do it. I do not doubt it for a moment. I see courage in your eyes, just as I did 100 years ago. I believe in you, Link.