Let's think about the role of onions in Louis Sachar's novel Holes. In the scenes of the past that are interwoven through the novel, Sam is the local onion man, and he firmly declares that an onion is
nature's magic vegetable. If a person ate nothing but raw onions, he could live to be two hundred years old.
Sam's “medical” advice to everyone is to eat plenty of onions, for those who take that advice seem to be cured of whatever malady they are experiencing. Sam keeps a secret onion field where “onions grow all year round, and the water runs uphill.”
That onion field proves to be lifesaving many years later when Stanley and Zero are lost in the desert. They find Sam's onion field, still filled with onions. They can eat all they want, and the supply does not run out, and those onions keep them alive. Stanley feels that the onions even make him strong. The onions also serve to protect the boys' precious water jars when Stanley slips and drops his sack.
What's more, onions actually save the boys from the horrible lizards that surround them down in the hole after they find the suitcase. The lizards crawl all over the boys, and Stanley and Zero expect the creatures to bite at any moment. These lizards are deadly, and with one bite, both of the boys would be dead. Yet the lizards don't bite, and it is because of the onions. After eating so many onions, the boys reek of them, and lizards are repelled by the smell. Sam has been proven correct in the end. The onion really is “nature's magic vegetable.”