Place the cake dough with the baking paper on an oven tray and prick the pastry base firmly with a fork. Spread over half the crème fraîche and top with the sauerkraut. Sweat the onions with the sugar in a non-stick frying pan over a medium heat for approx. 3 mins., distribute over the sauerkraut, season.
Bake for approx. 20 mins. on the bottom shelf of an oven preheated to 220°C. Distribute the cicorino over the tart and place the salmon on top. Serve with the remaining crème fraîche.