Green bean casserole, prepped to be refrigerated overnight and cooked tomorrow. Many people in the USA consider this an absolutely essential holiday traditional dish, most often served at Thanksgiving; I haven't had it before (not my own family's tradition) and am trying it for the first time now.
The secret ingredients are soy sauce and packaged dry fried onions. It's interesting to think about in what sense that's a "secret," because literally everybody who cares to make this dish, knows.