Possibly akin to the German expression translated "shit with mustard!" It exaggerates the repugnance of shit by evoking the image of eating it. With mustard, or with onions, either expression is amusing and ... well ... distasteful.
Found in "Ulysses" by James Joyce: "O! Mr Dedalus cried, giving vent to a hopeless groan. Shite and onions! That'll do, Ned. Life is too short."
by Little Ray d'Sunshine November 14, 2015