Heat the oven to 325°F/160°C. Bring a full kettle of water to the boil, and turn off the heat. (This is for the water bath later.)
Dissolve the sugar for the topping in a saucepan with ¼ cup/60 ml water, and boil until it turns a dark, fragrant, liquid caramel. Pour into the cake tin and swirl to coat the bottom evenly. Set aside.
Pour the milk into the same saucepan, so that it laps up the caramel on the bottom and sides. (This is the most brilliant trick for flavour!) Split the vanilla bean, scrape the seeds into the milk, and add the pod. Heat to the boiling point, turn off the heat, cover, and set aside to infuse 10 minutes. Meanwhile, beat the eggs with the sugar. Add the nutmeg, if using. Once the milk has infused, whisk it into the egg mixture.
Strain the custard over the hardened caramel and set the pan in a larger dish or roasting pan. Pour the boiled water into the bottom pan to come half-way up the sides of the flan. Transfer to the oven and bake until set, 45 minutes to an hour.
Remove the flan from the oven and from the water bath. Run a knife around the outside edge to loosen the flan. Cool completely – even chill, if you like. To serve, flip the flan onto a plate, pour over any caramel remaining in the pan. Serve in wedges.