By insisting upon his eating nothing else

[Mher]

Dear friends

Could you, please, paraphrase the underlined part. I cant understand how they managed to cure the patient.

"He was a troublesome patient; and we had much ado to keep him within bounds. For a long time he would eat nothing but thistles; but of this idea
we soon cured him by insisting upon his eating nothing else."

The patient was eating only thistles, and the cure was that they insisted that he continued to do so—that is how I read the sentence.

[teechar]

For a long time he would eat nothing but thistles

I take that to be an exaggeration. In other words, he did eat other food but pretended that he can only eat thistles. Or perhaps he thought he could fool them by eating only thistles for a while.

; but of this idea
we soon cured him by insisting upon his eating nothing else."
... they insisted that he continued to do so

That's right. They decided to give him only thistles, and he eventually relented and started eating other food again.