Never have I struggled so much to name an instructable. Other possible names:
Hot gold, liquid energy, hot lemonade -- the most accurate name, hot egg lemonade kind of makes it seem gross, rather than the most delicious, invigorating hot drink you have ever tasted. Liquid lemon tart describes its taste best -- though it does leave out the fact that this is a hot drink, perfect for nipping a cold in the bud. In my opinion it even beats chicken soup for the title of "best comfort food while lying sick in bed."

Step 1: Ingredients

You will need:

The juice from 1 lemon
1 egg
3 tablespoons of powdered sugar (aka confectioner's sugar)
Nutmeg and cinnamon (stick or powder) for garnish

Step 2: Instructions

Mix together the lemon juice, sugar and egg in a mug.

Slowly pour boiling water over the mixture, while continuing to beat it with a fork. Pour enough water to fill your mug. If you like your drink steaming hot, zap in in the microwave for a few seconds.

Step 3: Garnish

Grate some fresh nutmeg on top and garnish with a cinnamon stick.

You can also use pre-ground nutmeg and powdered cinnamon if you don't have the whole spices in your pantry.

This is a "vintage" recipe which I found in Henley's Twentieth Century Formulas, Recipes and Processes, published in 1914. I wrote all about this book in my Make Glue instructable.