(Mrs. Hannah), a name immortalised by the reputed saying in a cookery book, “First catch your hare,” then cook it according to the directions given. This, like many other smart sayings, evidently grew. The word in the cookery-book is “cast” (i.e. flay). “Take your hare, and when it is cast” (or cased), do so and so. (See Case, Catch your Hare.)
“We'll make you some sport with the fox ere we case him.” —
Some of them knew me, Else had they cased me like a cony.
Beaumont and Fletcher: Love's Pilgrimage, ii. 3.
First scotch your hare (though not in Mrs. Glasse) is the East Anglian word scatch (flay), and might suggest the play of words. Mrs. Glasse is the pseudonym which Dr. John Hill appended to his Cook's Oracle.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894