I'm looking for a word which describes the instance of a thing that causes you to become interested in that thing more generally.
For example, perhaps you are a person who has never cared much for sandwiches, then you have a sandwich that you find particularly delicious. Now you love sandwiches. You live for them.
"That was the NOUN/ADJECTIVE sandwich. Now I'm a connoisseur."
The best word that I can find is "gateway," as in a "gateway drug," or in this case a "gateway sandwich." But I think this meaning is only colloquial. I don't see gateway having this definition explicitly in any dictionary.