Ummm... my grandma did the same. Only it wasn't just onions and mustard. She added cayenne pepper, black pepper, lots of salt, and who knows what else. The interesting thing is, she didn't do it because they were poor. She did it whenever she felt a cold coming on. And truth be told, I don't remember her coming down with a cold very often, and usually it was when she was "out visiting" (i.e. family, week-long stays) and didn't have access to the right ingredients. Some of these strange recipes have a LOT of history and experience behind them...
I'll take the cold over onion sandwiches, though.
I'll take the cold over onion sandwiches, though.