Rinse, peel and cut the rhubarb into 2 cm (approximately 3/4 inch) long pieces. Mix with 2 tablespoons sugar and set aside.
Preheat the oven to 120°C (approximately 250°F) top and bottom heat.
Bring the milk and the cream to a boil, then remove from heat. Mix the egg yolks with the vanilla sugar, lemon zest and sugar until smooth. Gradually stir the hot milk into the egg cream, stirring constantly. Pour the cream into ovenproof ramekins and place the rhubarb on top. Place on a baking pan with high sides or in a baking dish. Pour in enough water to reach 2/3 up the sides of the ramekins. Bake for about 1 hour, then sprinkle with the brown sugar and preheat the broiler. Caramelize for about 5 minutes, until the sugar is light brown.