The answer to this question can be found in chapter 40 of the book. The chapter starts out in the present with Stanley digging up a root. We are not told what the root is, but the flashback that follows immediately indicates to readers that the root is an onion root. The flashback takes readers back about a hundred years to Green Lake. Mrs. Tennyson is running in her night clothes, and that is a sight to be seen because she is normally a very proper lady. She runs up to Sam and Mary Lou, and states flat out that Becca's recovery is thanks to Sam.

"Gladys Tennyson was all smiles. 'I think she's going to be all right. The fever broke about an hour ago. Thanks to you.'"

Sam tries to give the doctor some credit, but Mrs. Tennyson isn't having any of that nonsense. She doesn't trust the doctor at all. She reiterates that Sam and his onion tonic cured the fever.

"'It was your onion tonic,' said Mrs. Tennyson. 'That's what saved her.'"

Before leaving, Mrs. Tennyson then buys a heap of onions from Sam.