Until almost the 19th century, rhubarb was generally considered a medicinal plant. It entered the gardens of the Finnish gentry in the 18th century and passed into general use in the 19th century. Rhubarb was later used for jams, sweet dessert soups, drinks and pastries. Growing in gardens across Finland, rhubarb has a high season from early summer until the end of June, extending into August.Turnover cakes are layered in reverse order – the fillings are put in the dish first, and then the dough. After baking, the cake is turned upside down on a tray. The French Tarte tatin is possibly the most famous turnover. One story depicts its semi-accidental creation by the Tatin sisters in the 1880s. As with most turnovers, fruit or berries laid at the bottom of the dish caramelise with sugar during the baking, creating a delicious sweet coating on the cake.
Filling: Peel the rhubarb and cut it into 1 cm long pieces. Melt the fat on a flat cake mould or on a clean cast-iron pan. Add the brown sugar and rhubarb cubes.
Pastry: Beat the fat and sugar together until creamy. Add the eggs one at a time. Mix the baking powder and vanilla sugar with the flour. Add them and lemon juice (and grated lemon skin). Pour the pastry over the rhubarb filling.Bake at 175°C for about 45 minutes. Turn the tart upside down on a tray and serve it either warm or cold.