Liver.
Fresh liver. Lamb liver tastes fine when thinly sliced, seared in butter, duck fat or ghee, and well salted. Some form of allium plant, if tolerated, is a good side-by-side, forming that English, as in England cuisine beast, liver and onions.
Highly nutritious. Is there such a thing as a liver ‘fast’? Outwardly it seems immediately self limiting. The amount would need health limiting depending on Vitamin A requirements that could be over done (unlikely).
Anyway, I was chopping frozen free-range lamb liver (for some reason labelled as ‘fry’ in the local supermarket parlance), I was chopping it into small cubes for my 13 year old cat named ‘Boots’ - Nancy These Boots Were Made For Walking, is a bit too much to call out when she is sleeping in a cupboard. I want Boots to live for ever, so I feed her lamb liver with her beef. She looks and acts like a kitten.
Then I started popping frozen liver cubes in my mouth and washing them down with a sip of water. Like I needed them.
More liver: https://www.ketogenicforums.com/search?q=Liver
Chicken pâté often is sealed with butter or duck fat on top… But if I eat liver, that’s a very lean meal and only a single course among many in my meal. I can inhale 400g easily so I do need to limit myself a bit painfully… So cheap, so good, I could half-live on liver (for a short while, at least) if it wasn’t so super nutritious 
