Preheat oven to 325F. Place 4 small ramekins in a roasting pan.
Add the 1/2 cup sugar and 2 tablespoons of water to a small saucepan. Bring to a boil over medium-high heat on the stovetop until it turns a light to medium amber colour. DO NOT STIR.
1/2 cup (100g) granulated sugar, 2 Tbsp water
Remove from heat and immediately pour the caramel evenly into the ramekins.
Whisk together the egg, egg yolks and sugar.
1 large whole egg, 2 large egg yolks, 1/4 cup (50g) granulated sugar
Whisk in the rum, vanilla, and nutmeg in a large bowl.
2 Tbsp rum or bourbon, 1 tsp vanilla extract, 1/2 tsp nutmeg
Gradually whisk in the warm milk. Pour the mixture through a fine sieve and fill each ramekin to the inside line or 1/4" to the top.
1 & 1/2 cups (360ml) warm milk
Transfer the roasting pan to the oven and fill halfway up the side of the ramekins with boiling water.
Bake for about 45 minutes or until a toothpick inserted into the center comes out clean. The nutmeg will settle on top when baked, but not to worry, no one will see it when it's flipped over.
Carefully remove the ramekins from the hot water with tongs. Let cool to room temperature and chill in the fridge (loosely covered or in a large container with a lid) for a minimum of 3 hours or preferably overnight before serving.
When ready to serve, run a thin knife along the inside edge of the ramekin. Place a dessert plate on top and invert both. Give it a gentle shake to release the custard and caramel sauce onto the plate.
Notes
Crème caramel can be stored loosely covered or place in a large container with a lid in the fridge for up to 3 days.