Preheat the oven to 220°C, 425°F, Gas 7. Spread the mozzarella evenly over the pizza base. Place all the mushrooms into a bowl. Add the chopped chives then the truffle paste, Calabrian olive oil, garlic and half the grated Grana Padano. Mix well then scatter evenly over the pizza base.
Season with the Calabrian oregano, then transfer to a non-stick baking tray and cook in the preheated oven for 10-12 minutes until the pizza is crisp and golden. Remove from the oven and sprinkle over the remaining Grana Padano and the flat-leaf parsley.