whisk (though a fork'l do for this single serving)
Beat egg into milk. Beat in sugar; top with nutmeg.
The proportions of the recipe are entirely dependent on the flavourfulness (and freshness) of your eggs. Supermarket eggs are weaker, paler, older (and thereby more susceptible to bacterial contamination) than local, farm-raised eggs. Which is to say, don't try this at home unless you can rely on the freshness and sanitation of your egg supply. And if you can't trust the purity of your supply, please play it safe and purchase that industrial syrup your grocer labels ‘Eggnog.’