Lay 1 sheet of puff pastry into a 26 cm (10.2 in) pan and prick the base all over with a fork. Set aside.
Wash and dice 400g (14.1 oz) of zucchini. Peel and cut 300g (10.6 oz) of potatoes into roughly 1–1.5 cm (0.4–0.6 in) cubes.
In a large frying pan, heat 3 tablespoons of oil with 2 garlic cloves. Add the vegetables and stir.
Pour in 80ml (2.7 fl oz) of water (a scant half glass), cover and cook over medium heat: the vegetables should stew, not dry out. Stir now and then and add a tiny splash more water if needed.
Once the vegetables are soft, take off the lid and let any remaining liquid evaporate. Add 1 level teaspoon of salt, then taste and adjust.
Spread half the cooked vegetables over the puff pastry base.
Add 200g (7.1 oz) of diced mozzarella, then cover with the remaining vegetables.
Finish with 40g (1.4 oz) of grated cheese and 6 turns of the pepper mill.
Bake in a preheated static oven at 180°C (356°F) for 35 minutes (or convection at 170°C / 338°F).
Take it out and let it rest for 5 minutes before slicing.
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