Premade eggnog gives you a head start on this easy holiday dessert, while using maple syrup saves you the trouble of making caramel to coat the bottoms of the ramekins. You’ll need to make these flans at least 1 day (and up to 3 days) ahead, so the custard has plenty of time to infuse with the maple flavor.
Position 1 rack in middle of oven; position second rack in top. Preheat oven to 325 degrees F.
Whisk together the first 4 ingredients in a medium bowl. Place eggnog in a small heavy saucepan over medium heat; heat to 180 degrees F or until tiny bubbles form around the edges (do not boil). Gradually pour eggnog into egg mixture, whisking constantly. Strain through a fine-mesh strainer into a large 4-cup measuring cup or clean spouted bowl. Skim any foam from surface of custard.
Arrange 8 (5-ounce) ramekins in a 9-by-13-inch baking pan. Place pan on a rimmed baking sheet. (This will make it easier to maneuver the pan in and out of the oven.)
Pour maple syrup evenly into ramekins. Top evenly with custard. Skim any bubbles from surface of custard. Fill pan with hot water to a depth of 1-1/2 inches. Place pan on middle rack of oven. Place a second baking sheet on the top rack (this prevents the tops of the flans from browning). Bake 45 minutes or until set. Carefully remove ramekins from pan. Cool to room temperature. Cover, and refrigerate 24 hours or up to 3 days.
To serve, run a thin, sharp knife around the edge of each flan. Place a dessert plate upside-down over each flan. Invert to unmold flan, allowing maple to drizzle over flan. Garnish with nuts.