The Old Man's Diary is a key item from The Legend of Zelda: Breath of the Wild. Link can find it inside the Woodcutter's House located on the Great Plateau. Newer entries are added as Link progresses during the main quest "The Isolated Plateau". It also contains a recipe for Spicy Meat and Seafood Fry, but the Old Man will only add the complete one if Link helps him remember it to obtain the Warm Doublet.
"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."