Dried liver slices - I get turkey livers as my pup is raw fed and she has liver with her vegetables and supplements once a week - but you can use any liver. Slice it very finely, lay the slices on a sheet of greaseproof paper [or brown paper] and slowly dry in either a microwave oven on its lowest number, or in an oven, also on a very low heat.
You can then break the dried slices into whatever size you want.
You can then break the dried slices into whatever size you want.