Green’s Dictionary of Slang

half a sheep’s head boiled with onions.

1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1864, 1867, 1870		Hotten Sl. Dict.
1873		Sl. Dict.
1893		Western Mail 21 Dec. n.p.: A 'German duck,' or 'Field-lane duck,' is in ordinary eating-house mock heroic for a sheep's head stewed with onions.
1912		Dundee Courier 22 Jan. 8/1: ‘Welsh rabbit’ is just as [...] correct a popular name for toasted cheese [...] as ‘German duck’ is for sheep’s head in the East End.
1928		Bath Chron. 4 Aug. 25/6: The inquest ended with the luncheon hint, ‘German duck,’ sheep’s head stewed with onions, a favourite among German sugarbakers in the East End of London.
1947		Exeter & Plymouth Gaz. 24 Dec. 4/2: A sheep’s head stuffed with onions can be listed as ‘German duck’.