Cut the cucumber and onion into small cubes (approx. 1 x 1 cm).
Fry the onions in the tuna juice a little, e.g., in a wok pan. Add the tuna fillets, cut into small pieces, mix and fry a little. Add the cucumber and mix. Turn off the stove and let it cool down a bit. Mix in the grated cheese. Mix 4 eggs and 250 ml cream and mix in as well.
Brush a quiche / tart pan (approx. 32 cm) with a little oil, place the ready-to-bake quiche dough in the pan and prick the dough base several times with a fork. Pour the mixture into the mold, distribute it nicely and bake in the preheated oven (hot air 200 degrees) for about 35 - 40 minutes.