Drain the tuna, anchovies and capers, puree with the crème fraîche, season.
To bake the tarte flambées
Place the dough on two trays along with the baking paper. Spread the crème fraîche over the top, cover with the onions.
Bake each tray for approx. 15 mins. on the bottom shelf of an oven preheated to 220 °C (or both for approx. 15 mins. at 200 °C in a convection oven). Remove from the oven, top with the meat. Spread the tonnato sauce and rocket over the top.