Done to a turn
Cooked just right.
Since at least the end of the first Millennium, food, especially meat, has been cooked on spits. The English abbot and scholar, Aelfric of Eynsham, referred to them as 'spitu' in Latin Grammar and Glossary, circa 1000.
Spits were originally simple pointed sticks, which were used to hold meat near to a fire. Rotating spits were developed in the Middle Ages; initially turned by hand and later by various forms of powered mechanism.
The allusion in the phrase 'done to a turn', or 'roasted to a turn', is to food that had been cooked for the precisely correct number of turns of the spit. Both versions of the phrase date back to the 18th century and the 'roasted' form is first cited in a piece by an author called Mackenzie in Mirror No. 93, 1780:
"The beef was roasted to a turn."