Why Do Certain Foods, Especially Meats And Onions, Have A Similar Unpleasant Smell?

Hi recently i’ve noticed that certain foods have the same nasty smell and i’ve noticed it in all meats and onions they smell the same and i can’t quite point out what the smell is but it’s not a great one all good taste normal but the ones with meat will sometimes have an aftertaste of that same smell i’ve been smelling