Heat oven to 350° and butter a 2-quart baking dish.
Put milk and Grape-Nuts in a medium saucepan over medium-high heat and bring to a simmer.
Remove from heat, stir, and let cool 15 minutes.
In a medium-size bowl, beat eggs with sugar, vanilla, and salt.
Add the cooled milk and Grape-Nuts to egg mixture and stir well.
Pour into the prepared baking dish.
Sprinkle nutmeg over the top.
Set the baking dish into a deep roasting pan.
Place in the oven and pour water into the roasting pan, enough to reach halfway up the side of the dish.
Bake until almost set in the center, 50 to 60 minutes.
There should be a very slight jiggle when you shake the pan and a knife inserted into the center should come out clean.
Let the pudding set atop the stove for at least 20 minutes before serving.
Serve plain or with whipped cream.