The riddle
What Is the Best Key to a Good Dinner?
From The Handbook of Conundrums, 1915
Key hidden in the word
The last syllable of turkey sounds like key. Merriam-Webster gives the pronunciation as TER-kee, ending in the same sound as the word key. So the answer is the thing a dinner is built around and also a pun on the thing that opens a door. The question asks for a key, and the reply delivers one inside a word.
In a 1915 book
Edith B. Ordway's The Handbook of Conundrums, published in 1915, prints the question as "What is the best key to a good dinner?" and the answer as "Turkey." J. Gilchrist Lawson's 1924 collection prints it the same way. The wording has not changed in over a century.
Other kinds of key
A key can open a lock or name the starting note of a piece of music. The riddle does not use either sense directly, but they are the ones a listener tries first, and that is why the answer comes as a surprise. Key also means the crucial element, as in the key to success (Merriam-Webster), and the question seems to ask for that: the one thing a good dinner needs.
Showing the key
Hold up an empty hand as if holding a key, say the question, and then lower it when the answer arrives. A fun extension is to ask for other foods whose names end in a sound that matches an object. For a riddle that plays on keys in the other direction, try the riddle about a thing with keys that open no lock.