For the crust: Mix flour and salt. Rub in the butter until the mixture is crumbly. Add 3 – 4 tbsp ice cold water to form the dough. Do not knead. Shape into a flat disc and wrap in plastic wrap. Chill for 30 minutes.
In the meantime, heat up a large skillet over medium-low fire with 2 tbsp olive oil. Add onions and a generous pinch of salt. Let them cook for 20-30 minutes, stirring occasionally, until golden brown.
Roll out the dough and fit it into a greased pie dish. Cover the bottom with the grated cheese, then the onions, and the zucchini on top. Drizzle with olive oil and sprinkle with salt and pepper. Bake in a preheated the oven at 180°C/350°F for 25-30 minutes, or until the zucchini are tender.
