Mix the squash, potatoes, semolina, oil and salt. On a baking tray lined with baking paper, shape the mixture into a round base (approx. 30 cm in diameter).
Approx. 1 hr. on the bottom shelf of an oven preheated to 220°C.
Heat the oil in a frying pan. Sauté the shallot. Add the cherry tomatoes, cover and simmer for approx. 15 mins., season, crush with a fork. Spread the sauce over the roesti base. Mix the squash, spinach and oil, season with salt. Top with the mozzarella and vegetables.
Approx. 15 mins. in the centre of an oven preheated to 220°C.