Know one's onions

Anyone who cuts raw onions, knows the effect upon eyes and nose - irritation, violent sneezing and lachrymation.

Red onion

When - Like Cures Like - it ought to be a good remedy for coryza... So it is.

Onion causes coryza-like symptoms: catarrhal inflammation of mucous membrane; runny nose, acrid discharge, sneezing, smarting eyes, coughing, all worse in evening and in a warm room. 

Coryza-like symptoms - if similar enough to onion caused ones, are cured by red onion: Allium Cepa - as medicine.

The right medicine sought, is not merely to be familiar, but also is to be right — by virtue of its relation of similarity,  to the suffering sought to be cured; which is, primarily, a qualitative relation, that is refined by quantitative adjustments, only secondarily, as dose and potency of choice medicine.