Preheat oven (electric cooker: 225 °C/circulating air: 200 °C/gas: level 4). Clean and wash spring onions and cut them diagonally into rings. Mix sour cream and thyme. Season with salt and pepper. Drain the tuna and tear with a fork.
Unroll the dough and place it on a floured work surface with the paper facing upwards. Peel off the paper. Cut the dough sheet in half crosswise. Roll out one half thinly to a rectangle (approx. 25 x 30 cm). Place on a baking tray lined with baking paper.
Spread half the sour cream on it. Spread half spring onions and half tuna on each. Bake in a hot oven for about 5 minutes.
Roll out the second half of the dough in the same way, spread or cover and bake. Cut the tarte flambée into pieces and serve.