Mix the flour, olive oil, egg yolk, sea salt and 100 ml water to form a dough, knead for 5 minutes and leave to rest for a while.
Peel and halve the onions. Thinly slice the courgette and onions.
Roll out the dough very thinly and place on a greased baking sheet. Spread the sour cream over the dough and top with first the sliced courgette then the sliced onion, and season.
Sprinkle the Gorgonzola evenly over the tart.
Bake in a preheated oven on the middle shelf at 250 °C for 10 - 15 minutes and serve immediately.