You can substitute store-bought puff pastry if necessary. I used hothouse rhubarb, which is sweeter than rhubarb grown outdoors, so adjust sugar level accordingly.
Ready time: 4 hours, including chilling
250 g (8 oz) rhubarb
1/2 cup sugar
2 tbsp water
1 1/3 cup flour
2 tbsp sugar
1/4 tsp salt
1/2 cup butter
3 tbsp ice-cold water
1 tsp vinegar
2 large eggs
1/2 cup butter, melted
1/2 cup 35-per-cent cream
1 tsp vanilla extract
1 cup light brown sugar
3 tbsp flour
Preheat oven to 300F. Cut rhubarb to fit into an eight-inch square baking pan. Sprinkle with sugar and add water. Cover with foil. Bake for about 30 minutes or until tender. Cool completely. Cut rhubarb into 1/2-inch pieces and reserve. Reduce liquid in a pot on high heat until syrupy, about 2 minutes.
Combine flour, sugar, salt and butter in food processor. Pulse until butter is pea-sized. Remove mixture from food processor into a bowl. Stir in water and vinegar, mixing until dough holds together in a ball. You may need a little less or more water.
Flatten ball and wrap in plastic wrap. Chill 30 minutes. Roll out pastry on a well-floured surface to fit a nine-inch tart ring. Chill for 1 hour.
Place parchment paper over pastry and cover with dried beans. Bake 20 minutes, remove weights and bake another 10 minutes or until pastry is golden.
Mix all custard ingredients in a bowl with an immersion blender, or whisk together eggs, butter, cream and vanilla, then whisk into brown sugar and flour.
Lay rhubarb on tart base and pour custard over. Bake at 350F for about 40 minutes, or until tart is no longer runny in the centre. Allow to cool for 1 hour on a wire rack. Drizzle with reduced rhubarb syrup before serving.