In a pie pan, lay the tomatoes and sliced zucchini. Cook in the oven at 180 °C for 30 minutes.
In the meantime, prepare a pie dough and set it aside in the fridge.
After 30 minutes in the oven, the veggies should come out caramelized. Push them aside so that you can add a layer of sliced onions on the bottom of the pan. On the top, add the spinach leaves and the rolled out the dough as a last layer. Make sure the dough is pierced through with a fork to avoid air bubbles.