Walnuts were edible nuts found on Earth.
The Third Doctor likened the size of the stegosaurus' brain to that of a walnut. (TV: Invasion of the Dinosaurs)
One Christmas Eve, the Fourth Doctor snuck into Alistair Gordon Lethbridge-Stewart's house dressed as Santa Claus and left two oranges and a walnut in his Christmas stocking. (PROSE: Special Occasions: 3. Better Watch Out, Better Take Care)
When trapped in a subjective reality modelled upon the Alice in Wonderland book she had been reading and the salad she had been eating, Peri Brown encountered a pair of talking turnips who began reciting her a poem called "The Walnut and the Cauliflower." (COMIC: Salad Daze)
In 1840, the Eighth Doctor bought a walnut muffin whilst in London. It was knocked from his grasp when Penny Chapman ran into him. (COMIC: The Curious Tale of Spring-Heeled Jack)
To battle the effects of cyanide the Tenth Doctor needed, among other things, some sort of protein. Donna Noble found a jar of walnuts, which the Doctor wolfed down. (TV: The Unicorn and the Wasp)