Heat oven to 350° F. Butter a 2-quart baking dish or 6 six-ounce ramekins. Pour the Grape-Nuts into a large bowl and set aside.
Heat the milk in a large saucepan until it begins to froth and bubble gently at the edges. Remove from heat. Pour warm milk over Grape Nuts and let stand for 5 minutes.
Whisk together the eggs, sugar, vanilla, salt, and ½ teaspoon nutmeg in a large bowl. Pour the mixture into the milk and Grape Nuts mixture and stir to combine. Pour into the prepared baking dish or divide evenly among the ramekins if using. Sprinkle tip with remaining nutmeg. Place the dish(es) in a larger roasting pan or baking dish and transfer to oven rack. Carefully add enough hot water to the larger pan to reach halfway up the sides of the smaller dish. Bake until the custard is set, about 45-60 minutes. (If you are using ramekins, the baking time will be shorter: start checking after 30 minutes.) Allow to cool at least 30 minutes.
Serve slightly warm or chilled, topped with whipped cream and maple syrup, if desired.