Green’s Dictionary of Slang

corns and bunions n.

onions.

1930		(con. 1914–18) Brophy & Partridge Songs and Sl. of the British Soldier.
1960		J. Franklyn Dict. of Rhy. Sl.
1972		Dodson & Saczek Dict. of Cockney Rhy. Sl. 25: corns and bunions. Onions [...] also in expressions like: She knows her corns and bunions, i.e. She knows her stuff.
1992		R. Puxley Cockney Rabbit.
2002		B. Kirkpatrick Wicked Cockney Rhy. Sl.